Entities
Entities are basically everything you can place in a map that aren't static brushes. Entities can be brush-based, e.g. a door or a lift but they aren't just brushes anymore. The other kind of entity is the point-based entity, which is weapons, trees, enemies, health and so on. Anything that isn't a brush-based entity. They are placed with the RMB sub-menu and modified in the entity inspector, brough up with the N key.