Basically anything you can see or hear is defined by one or many datablocks - weapons, vehicles, particles, particle emitters, sounds, bricks, projectiles, playertypes, etc. Not all add-ons use datablocks, though, for example just about everything I make is just script, and uses no datablocks. If you start a server with an add-on enabled, you can scroll up in the console to see when it was executed, and it will say how many datablocks it uses.
