Wide angle cap vs wide angle lens

The cap is a plastic disk, usually black or silver, that snaps on at the end of the lens to keep the lens clean. Some lenses have two caps, one for each end when not attached to the camera.
Some cameras don't use a "loose" cap; those cameras generally retract the lens into the camera body and "close" a door in front of the lens.
Many, if not most, camera users do not use a cap (my guess).

The wide angle lens is generally an "optional" lens that can be used for group or scenery pictures when you want to shoot a "wide" picture. A "normal" lens is generally around 50mm (plus / minus 5mm). A wide lens is anywhere from 17mm to 24mm (or so).

I recommend obtaining a wide lens... with a cap.

Hope this helps!