Here are some:
IV = Individual Value = Random numbers(up to 32) added to every stat when you catch a Pokémon
EV = Effort Value = Read
here
Shiny = ... = A Pokémon in "wrong" color, that has a chance of 1 on ~8200 of appearing in the wild
PKRS = Pokérus = A sort of virus that also has a chance of 1/~8000 of appearing and doubles EVs
Crit = Critical Hit = An attack that does double the normal damage, has 8% chance on normal attacks, and 15% on attacks such as Slash, Blaze Kick and Sky Attack. You'll get the message "A Critical Hit!" if you manage to land one
KO = Status: FNT = The Pokémon has 0 HP left
ANNOYING ******* MOUSE!!!!!! = Pikachu = An annoying little electric mouse -.-
AR = Action Replay = A Cheating Device
GS = Game Shark = Another Cheating Device
Dual-Type = Pokémon with two types = Pokémons like, for example, Blaziken is BOTH Fire AND Fighting
Single-Type = Pokémon with only one type = Pokémons like, for example, Alakazam is ONLY Psychic
400/Double/4x Weakness = 400% weakness = Attack damage multiply gotten by attacking a Dual-typed Pokémon with something both its types are weak to(like Heracross(Bug/Fighting) -> Flying)
200/2x Weakness = 200% weakness = Attack damage multiply gotten by attacking a Dual-typed Pokémon with something one of it's types are weak to and the other is not resistant against, or a Single-Typed Pokémon with something it's weak to
== GAMES ==
R/B/Y = Red/Blue/Yellow, first generation games
G/S/C = Gold/Silver/Crystal, second generation games
R/S/E = Ruby/Sapphire/Emerald, first of the third generation games
FR/LG = FireRed/LeafGreen, 1st generation remakes, second of the third generation games
D/P/Pt = Diamond/Pearl/Platinum, fourth, and most recent generation games
Hope that helped!