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From HalfLife 2 Knowledge Base

In the Hammer Editor, an "entity" is essentially a dynamic object. Just about any object in your map that has more complicated behavior than simply existing (as a set of geometric boundaries) is an entity.

There are three types of entities, point-based entities, model entities, and brush-based entities.

Brush-based entities were once brushes and have been converted into a brush-entity ("Tie to Entity", or Control+T when a brush is selected). A brush-based entity is connected to a brush which located it in physical space within the game world. This can be an object in your map, such as a door or crane, or it can represent an invisible area such as a trigger area for an event.

In contrast, a simple Brush is a far more simple object, used primarily to represent the geometric bounds and large (static) set-pieces of a map.

A point-based entity exists only at a point in space, but has no volume and is not inherently linked to any physical objects in your game world. Point-entities are often used simply as points to describe how and where an effect should act. For example, env_beam effects use point-entities to describe their effects.

Model-based entities are perhaps the most obvious and intuitive of the three types. These include such notable items as weapons and armor. Typically, these are special, game/mod-specific types of items.

See also:

• Brush
• Hammer Editor
• "Creating Entities" (Valve Hammer User's Guide)
• Entity Guide