The concept of clipping and masking is pretty simple. Hide certain parts of elements and show others. The actual difference between them is pretty simple too. Clipping is always a vector path, where inside the path is visible and outside the path is not. Where a mask is an image, is treated as a grayscale image where the black parts mask the image to transparency and the white parts let the image through.
Implementing clipping and masking isn't particularly easy though, as the browser support for it (and all the little ins-and-outs) is quite varies. We try and go through all of it in this screencast, struggles and all.