A face is a three-sided surface unit that represents a physical surface in 3D modeling. A face is also called a triangle or a tri. Compare with a polygon, which is a more general term for any surface unit with three or more sides.
Technically, the term face refers only to a three-sided polygon, but in practice these terms are used interchangeably.
A face should not be confused with a shape. A shape is just a line with no surface for rendering. A face represents a physical surface that can be rendered.