Video Class, in this case, means moving video from a web camera or digital video camera to another device like a display, or PC, or game console.
UVC is built into Windows, Mac OX, Linux, FreeBSD, and the PS3.
Simply, UVC makes it easier by standardizing video drivers. So any Video Camera could use the standard UVC driver in those platforms. The Video Camera maker only needs to worry about the embedded firmware in the Video Camera itself.