Terms to Know

Metagame - This is the term used to describe the competitive battling arena. As per Wikipedia, which states it quite clearly, a metagame "is a broad term usually used to define any strategy, action or method used in a game which transcends a prescribed ruleset, uses external factors to affect the game, or goes beyond the supposed limits or environment set by the game." It goes on to describe it as the game universe outside of the game itself. Essentially, this means that if you're using in-game Pokemon, you probably aren't playing the metagame. If you're familiar with Smogon, the Pokemon metagame is what they deal with.

Set - Also moveset or niche, this term is used to describe a set of factors that include the EVs, IVs, the moves on the Poke, the base stats of the Poke, the ability of the Poke, the item on the Poke, the nature of the Poke, and the strategy that the Poke is trying to accomplish. There are a multitude of archetypal sets, such as walls, sweepers, and spinners, but many Pokemon are able to run more specialized and unique sets, such as Magnezone's steeltrapper set.

Wall - A set that is designed to "wall" opposing Pokemon. Usually, this is characterized by significant stress on defenses, residual damage rather that attacking outright (such as Toxic, hail, or Steal Rocks), and the ability to heal off damage with moves such as Roost or Wish. Note that these are not prerequisites, but general guidelines. There are many walls without the ability to heal, for instance, and many walls that have the ability to inflict large amounts of damage. You can have physical walls, such as Skarmory, special walls, such as Blissey, or just general walls, like Shuckle.

Sweeper - This set is basically the opposite of a wall. Rather than trying to wear down the opponent over time with residual damage, the goal of a sweeper is to wreak havok and totally whelm the opponent with sheer strength. This usually necessitates a wide range of offensive moves, stress on either attack or special attack, and speed. Because of these emphases, sweepers are often times much more frail than the average Poke. A branch of sweepers are the set-up sweepers, where the Pokemon runs some sort of move that boosts their own stats in order to pull off what is termed a sweep, a string of subsequent KO's by the same Pokemon.

CB - This is short for Choice Band or, less often, the attack Charge Beam.

CS - This is short for Choice Scarf or Choice Specs.

These terms are usually used as a sort of prefix attached to a Pokemon's name, which usually implies what set it's running. For instance, one very common one is CB Scizor. This means that you have a Scizor running a Choice Band, and usually the moveset for a CB Scizor is Bullet Punch, Pursuit, U-Turn, and Superpower.

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Spinner- A pokemon that possesses the move Rapid spin. The pokemon's main goal is to clean your side of stealth rocks, spikes and the like. It is almost essential for the good use of flying bugs, fire flying and ice flying types to have a spinner on their team.

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Tank- A Pokemon that is made to withstand many hits, and easily win fight. Tanks normally are EV'd highly in HP, and misc. in both Defenses and one type of attack. Tanks normally have few weakness'.

Also note that the difference between a wall and a tank is that a wall is meant to take hits and stall most of the time while a tank can still dish out good damage and take hits. Tanks will usually utilize Curse, Bulk Up, or Calm Mind, but are not fast enought to sweep

Cleric - One of the least used sets, at least in my experience. A cleric's primary function is to heal the rest of the team. Moves like Heal Bell, Aromatherapy, Wish, Healing Wish, and other moves that either restore HP or eliminate status conditions are the key to a cleric. Most often, clerics are focused on livability so that they can come in multiple times and do their healing. Due to this investment in bulkiness, they are rarely focused on doing direct damage.

Bulky - This is a term used to describe a Poke that has an unusually high investment into its HP and defenses.

Spinblocking - This is the act of preventing the opponent from rapid spinning. How is this done? Send in a Ghost type. Because Rapid Spin is a normal type attack, it is not capable of hitting ghosts, and, therefore, both the damage it inflicts and the secondary effects in incurs are rendered useless.

Lefties - This is the shorthand term for leftovers. You'll find a lot of abbreviated terms, but most of them are pretty self-explanatory, e.g. sash. By the way, are there any terms that people would like to request?

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These are much more universal terms that apply to more than just the Pokemon metagame, but I've heard them used in competitive battles on YouTube and such, so I figured I'd explain them, just in case.

Nerfed- Used as either an adjective or a verb, nerfed is used to describe something going from a position of high power to one of a lower power. For instance, with the introduction of the type split in Gen III, Alakazam was nerfed due to the fact that it could no longer effectively use the elemental punches, greatly reducing the versatility of its movepool. You could also say that if your CB Scizor gets burned, then it has been nerfed, since it has been reduced in attack power.

Broken- This terms means pretty much the exact opposite of what one might originally think: the Poke iteself is not broken in any way. On the contrary, the Pokemon itself breaks the game, meaning it is too powerful to make the game fun to play. Ubers are, practically by definition, broken, which is why they have their own tier to mess around with.

Hax - hax are basically any luck-based events that occur in a battle. For instance, parahax refers specifically to getting fully paralyzed. There's also flinch hax, crit hax, freeze hax, and, while I don't think there's a specific term for it, you can get haxed through missing with a move. These simply refer to when a non player-based factor changes the flow of the battle.