Common Rules
Note that by no means is this necessarily a comprehensive list.
Level 50 All / Level 100 All / Single Level 50 / Single Level 100 / Double Level 50 / Double Level 100
People new to the concept of battling using Wifi or Pokemon Battle Revolution should become familiar with these rules right now. When you battling using these in-game rules, all pokemon will be at the same level for the battle. This allows people to compete on an even playing field, allowing new trainers and old to battle.
No Legendaries
This is a very common rule which is very simple to follow: just don't use any of the legendary pokemon!
A few pokemon to clarify right off the bat:
Rotom - Is not classified as legendary
Phione - Is referred to as legendary by Nintendo (making it the weakest legendary)
No Ubers
This is another very common rule. It basically means to not use any pokemon currently classified in the Uber tier. This includes broken and powerful pokemon. (This is mostly ) Pokemon that are put into the Uber tier are known to be broken and severely change general gameplay.
The consequences of this rule is that many legendary pokemon become viable to use. There are legendary pokemon that you're allowed to use:
-Articuno
-Zapdos
-Moltres
-Entei
-Suicune
-Raikou
-Ho-oh
-Celebi
-Regirock
-Regice
-Registeel
-Jirachi
-Speed Forme Deoxys (recently unbanned)
-Uxie
-Mesprit
-Azelf
-Heatran
-Regigigas
-Cresselia
-Phione
-Shaymin
Pokemon in the Uber tier that you're not allowed to use:
-Mewtwo
-Mew
-Lugia
-Latias
-Latios
-Kyogre
-Groudon
-Rayquaza
-Normal-Forme Deoxys
-Attack-Forme Deoxys
-Defense-Forme Deoxys
-Dialga
-Palkia
-Giratina
-Manaphy
-Darkrai
-Arceus (all forms)
Wobbuffet and Wynaut might have just been bumped down to the OU tier. (They were in Limbo, meaning no official decision had been made.)
Evasion Clause
Moves such as Double Team and Minimize are commonly frowned upon because they severely remove skill from battles/designing to place luck as the number one factor determining victory. Attacks which are designed solely to lower the opponent's accuracy might fall under this clause as well.
This does not necessarily include items and abilities which increase evasion, but noteworthy trainers would be wise to prepare for situations where the opponent has a more strict evasion clause, just in case.
OHKO Clause
Another class of moves usually avoided are the super-low-accuracy, kill-in-one-hit moves such as Fissure or Horn Drill. Because they do not work based upon damage calculation and introduce a huge luck factor into battle, they are commonly banned from battles as well.
Moves such as Endeavor or Destiny Bond are generally not included in this since they are purely strategic moves which require prediction, not luck.
Specie Clause
People commonly do this, anyways, but it's worth a mention. It simply means limiting yourself to one of each pokemon in your party.
You might be able to force this condition on Pokemon Battle Revolution.
Loose Specie Clause
Like the Specie Clause, except pokemon which are clearly out classed by commonly used pokemon aren't restricted. For example, a team of all Magikarps would be okay under this clause but a team of all Gyarados wouldn't.
Item Clause
To clarify, you are already forbidden from using any items from your bag by the rules of the game, itself. However, you should already be familiar with the fact that a pokemon can hold an item to use in combat.
Item Clause is like the Specie Clause ; you're only allowed to use one of each hold item. This means you are not allowed to have two pokemon holding Leftovers, or an entire team that uses Choice Scarf.
This can be enforced using Pokemon Battle Revolution rules.
Loose Item Clause
This is similar to the Item Clause except more than one item is allowed. This is a rule made up for our Pokemon League. It limits teams to a max of two of any one item.
Sleep Clause
The Sleep Clause is simple ; any team is only allowed to have one sleeping pokemon at a time. After one of your pokemon is put to sleep, the opponent cannot put any more of your pokemon to sleep. (There might be similar Paralyze, Burn, Freeze, and Poison clauses, maybe bunched together as a Status Clause rule.)
Unlike the Item Clause and Specie Clause (which affect how you design your team) this common rule changes what can happen during battle. For this reason it's not common in normal pokemon battling.
It might be possible to enforce this on Pokemon Battle Revolution. Shoddy Battle, a pokemon battle simulator, has the ability to enforce this clause.
Ladder Clause
I've seen mention of this but don't know for certain what it means. It's a Shoddy Battle rule so there's a couple possibilities I've come up with.
It could mean that the results of the match should be recorded for your overall score on Shoddy's server.
It could mean that you must strictly adhere to the tier concept.
Or, it could be a combination of the two. Unfortunately, at the moment, I don't know.
Strict Damage Clause
This appears to be another Shoddy Battle rule. This one simply changes how damage is displayed when you do more than enough damage to take your opponent down in one hit. For example, if the Shoddy Battle program would normally said "you did 230% damage" it would, instead, bring that down to "you did 100% damage."
Legit Clause
This currently means that the trainers must use pokemon obtained / created legitimately. Pokemon which are obtained by using a cheating device or created / modified using a cheating device are explicitly not allowed.
Team Size Limit
Teams are not allowed to have more pokemon than the given amount (although they could have less.)
Theme Limit
Team must follow a given theme. The most common themes are element-typed. This doesn't require that all members of the team be of that type. A few could be differently-typed as long as the overall theme is still followed.
Terminology
Note that I am only human and these terms are subject to misinterpretation on my part. Consider these to be a guideline.
Sweeper
A sweeper is a pokemon designed to take down as many of your opponents pokemon as it can, as fast as it can. By necessity, these pokemon have high attack stats and high speed stats. They tend to fall into three categories:
-Physical Sweeper (Uses primarily physical attacks with its Attack stat)
-Special Sweeper (Uses primarily special attacks with its Special Attack stat)
-Mixed Sweeper (Uses both physical and special attacks)
Tank
A tank is a pokemon with a high attack stat and good defensive stats, often at the cost of speed. Usually, they can be sent in on an opponent's sweeper, take the hit, and deal lots of damage in response.
Wall
A wall is a pokemon designed to completely stop attacks from working. Their primary purpose is to stop a sweep or to slow down the game in your favor. Like sweepers, there are physical, special, and mixed walls. Examples of a couple common walls:
-Skarmory (its steel type allows it to resist many types, Flying removes its weakness to ground and reduces the danger of fighting, and its Defense stat is fairly high.)
-Blissey (the pokemon with the highest HP in the game, it also has a really high special defense stat, making it incredibly difficult to take it down with special attacks.)
Revenge Killer
A revenge killer is a pokemon you send out after one of your pokemon has fainted in order to "get revenge", usually by simply finishing the opponent's pokemon off (if they're already a bit weakened.) A really good example of a revenge killer is Dugtrio. His high speed allows him to outrun a lot of pokemon and his Arena Trap ability prevents all opponents on the ground from fleeing, allowing you to deal the final blow to a dangerous pokemon before it's saved for later.
Base Stats, EVs, EPs, IVs
These are the values that are used to calculate a pokemon's actual stat (ignoring level for the moment.)
-Base Stats determine which pokemon will be fast, slow, etc. For example, Gengar has a base stat speed of 110 while Jolteon will have a base stat speed of 130. This means a Jolteon will be naturally faster than a Gengar. Steelix, on the other hand, has a base stat speed of 30, meaning it'll be really slow.
-EVs/EPs are short for "Effort Values" or "Effort Points" ; this represents training that you've done to your pokemon. It can add up to 63 points to a single stat. (It can only add 127 overall, though.)
-IVs are short for "Individual Values" ; this represents the random component of a pokemon's stat. It is determined at birth / creation and can add between 0 and 31 points to a single stat.
EV Spread / EV Training
Since EVs are determined by training, you can plan them out for your pokemon and train it from birth to add to the stats you want. Where you plan to put your EV points would be an EV spread, and doing the work to actually give those EVs to your pokemon would be EV training.
IV breeding / IV battle
Since IVs are determined at birth, IV breeding is the process of breeding pokemon until you've maxed out the IVs for a few stats. However, at level 1, you can't tell what a pokemon's IVs actually are. One way of checking IVs is to use the before-mentioned Single Level 100 wifi battle rule to force all pokemon to level 100. Checking IVs this way is called an "IV battle".
IV Calculator
An IV calculator attempts to guess what the IVs for a pokemon are. In general, you plug in the stats for your pokemon and the calculator will make an approximate guess. These can be useful even if you can't use IV Battling.
Hidden Power (HP)
This refers to the move "Hidden Power." The actual power and the type of the move are determined by IVs and it's not unusual to see IV calculators also take a guess at what the Hidden Power type and power will be. This move is important because often pokemon cannot learn moves of a certain type that would be useful to them.
Confusingly, the move is commonly abbreviated to HP, which sounds very similar to the HP stat. For example, "Hp Ice 68" is short for "an ice-type, power 68 Hidden Power."
Hax / Hacks
Variations of the usage of this word exist, including the conventional "he's cheating" terminology, but another usage exists: an acknowledgment of an uncommon battle condition occurring. This is better explained by example:
Quick Claw Hax ; refers to the Quick Claw item activating at a critical time. (1/16 chance of this happening.)
Crit Hax ; "crit" is short for "Critical Hit" and refers to a critical hit occurring.
Miss/Whiff Hax ; refers to an attack missing (especially when the chance of the attack hitting is quite high.)
Paralyze Hax ; this is an example of a secondary effect. If Thunder Wave was used to paralyze the opponent, that wouldn't be Paralyze Hax since that's the expected behavior of Thunder Wave. However, moves like Thunderbolt only have a 10% chance of paralyzing. Paralyze Hax refers to situations such as when Thunderbolt successfully paralyzes the opponent.
In general, it may seem like such terminology is quite silly and trivial to bother mentioning here, however terminology such as can be valuable for spur-of-the-moment battle analysis. (Example: "You've got her Jirachi unless she gets Quick Claw Hax.")
Tier
Clearly not all pokemon are created equal, and some are pokemon you'd expect to see more often than others in competitive play. Tiers are a way to describe this and it allows for the exclusion of certain groups of pokemon so that other pokemon can see battle time.
Here's a list of tiers as defined by Smogon:
Uber - These pokemon are powerful and using them severly changes the way the game is played. The most powerful legendaries have been put into this category. This is also a way to ban pokemon from the OU tier.
OU - Stands for "Over-Used" ; these pokemon are used a lot in competitive play. Usually, a pokemon is OU because it has tremendous value to a team.
BL - Stands for "Border-Line" ; these pokemon are still common in competitive play, even if they're not as common as OU pokemon. A pokemon that's in BL still has good uses for a team but would be used to fill niches. This is also a way to ban pokemon from the UU tier.
UU - Stands for "Under-Used" ; these pokemon are still used by people, but they only show up for competitive play very rarely, save for "UU battles." Some of these pokemon can compete in the standard OU/BL teams.
NU - These pokemon are "Never-Used" ; they're rare to use because they usually have more powerful evolved forms that simply out-perform them in every way, shape, and form.
NFE - Although not exactly a tier, pokemon which aren't rated because they are "Not Fully Evolved" are put in here.
Boltbeam
This primarily refers to the combination of the two attacks Thunderbolt and Ice Beam, but it is common to refer to any electric/ice combination attacks this way. This terminology sees popular usage because the two types are resisted by few pokemon:
Magnemite, Magneton, and Magnezone (Steel + Electric)
Chinchou and Lanturn (Water + Volt Absorb)
Shedinja (Wonder Guard)
Some currently unnamed contenders:
Fire + Dragon (only resisted by Heatran )
Ground + Ice (only resisted by Surskit, Shedinja, and Bronzor+Bronzong)
(I imagine that there's more terminology to come.)