When you select one face it shown as kind of grey texture on the face itself with orange edges. Then if you select two or more faces others will be painted full orange (vertices, edges and faces) and there still will be one with the grey texture. It appears to me as some kind of key element for editing.
It is the "active" face, which usually is the last selected face.
You can find more information on this topic in the blender manual under "Selection". Here's the relevant quote:
Selected faces and their selection point are drawn in orange, unselected faces are drawn in black, and the active or last selected face is highlighted in white.