Faces

A face is a side of a brush. A solid cube, for example, has six faces: top, bottom, left side, right side, forward side, and back side. A hollow cube has twelve faces: six outside and six inside. Each face has attributes that you can change from the Face Attributes dialog box. These attributes control the texture that is applied to the brush, whether it's a mirror or emits light, and other details about how the face will appear when it's rendered in the game.

Note that cut brushes, although invisible themselves, create new faces when they cut into visible geometry. These faces belong to the cut brush, and as such are modified by changing the faces on the cut brush, not on the brush that was cut.