Brush entity
A Brush Entity is an entity type in the 
GoldSrc and 
Source engines, created by tying an entity to a BSP geometry brush in the map, giving the brush a specific effect or ability defined by the entity tied to it. (The default shortcut is Ctrl + T after selecting a brush)
Purpose and Uses
Brush entities are used for entities that require an area or trigger zone to be touched, hit, or used in order to perform a specific task. Another use of brush entities is to create a physical object that can move, such as an elevator, or a door (can be made by many brushes selected together and tied as a whole to a brush entity, forming a brush model). A great example of a brush entity that implements both of these designs is a func_button, which requires a brush to specify the area the player must press, touch, or shoot to activate the button; the button can be made using any shape of brush, or even a group of brushes, and can be textured to give the appearance of a real button. Brush entities such as the func_button have special features that affect the physical brush they're tied to, such as moving the brush in a general direction when activited, giving the effect of the button being pushed in and then popping back out. Another example of a brush entity is a trigger_multiple which can fire some outputs once the player walks into its area of effect (the volume of which is defined by the volume of the original brush used to make the trigger).