Most content packs contain a mixture of new sets, props, costumes, characters, animations, sounds and so on. Some are more specialised.
The current content packs include everyday clothes, hairstyles, and homes, cops, criminals, musicians, aliens, monsters, vehicles, buildings, music, special effects, spaceships, among other things.
The pages for the individual packs will give you a good idea of what's in each one and what you can do with it.