Entities are elements of a
map that perform certain functions. While
brushes make up the structure of a map, entities define all aspects of a map that make a map playable and interactive.
In
GoldSrc, there are two different types of entities:
point entities and
brush entities. Brush entities are brushes that perform functions beyond being the static geometry of a level, such as doors and triggers. Point entities are used to manage all other game logic, including playing sounds, defining level starts/ends, placing enemies and models, spawning items, and much more.
Every entity has a dictionary of keys and values which holds information about the entity such as its class name, name, offset in the map, the facing direction, and various entity-specific values. In the level editors, there will be a dedicated UI panel displaying these entries in a listbox for the currently selected entity in the editor. As for what entity classnames and key values are available for any particular game, most level editors read
FGD file(s) set up for the game.
Each
GoldSrc and
Source game has a different set of entities available to it. Specific entity information can be found in the respective game's entity guides, either official or third-party. TWHL hosts
Entity Guides for some GoldSrc games like Half-Life and Counter-Strike, though not in any official or authoritative capacity.
For some reason, GoldSrc prohibits an entity having the same
targetname
value as its
classname
(e.g. you can't have a
multisource
entity be named
multisource
). You will see warnings about this in the various entity guide pages, but it applies to
all entities, just to be clear.