The difference between clipping and masking
Masks are images; Clips are paths.
Imagine a square image that is a left-to-right, black-to-white gradient. That can be a mask. The element it is applied to will be transparent (see-through) where there is black in our gradient mask image, and opaque (normal) where there is white. So the final result will be an element that fades in from left to right.
Clips are always vector paths. Outside the path is transparent, inside the path is opaque.