CBaseTrigger

is a base class for all trigger entities. Usually triggers are tied to a brush that causes them to react in some way when another entity enters that volume.

Contrary to popular belief, triggers may also be point entities that simply specify a volume with their Minimum Bounding Box Size  and Maximum Bounding Box Size. Defining the volume this way is easier to do for entities that are created post-compile, but usually the method isn't sought by level designers because brushes are easier to manipulate.