(from the Minecraft Wiki)
mods: these are basically changes that you make to the game to make gameplay more fun or more suited to your needs.
features: these are components of the game such as switches, lights, animals, blocks, or anything else that resides in the world of mincraft.
map structures: These can be anything from a small house made of dirt blocks to a massive fort made of stone. (villages, strongholds and nether fortresses)
skins: These are particularly fun piece of the game due to the fact that they change the appearance of your character or things in the game.
NPC: these are the Non Playable Characters, these are AI in the game that you can interact with but not play as in the game.
Texturepacks: These are packs made by groups or individuals to change the look of minecraft. Texture packs do not add new items, mobs or blocks to the game.
SMP SSP: The first S stands for survival, the MP and SP stand for Multi Player and Single player.
Spleef: A popular game in SMP, in which the player has to dig the blocks under his opponent to kill said opponent.
PvP: This is the shortened version of 'Player vs Player'. This is a useful term to know in SMP.
