please have a look at my video linked below (Clipping_vs_Masking.mov).
In (A.) the pink rectangle (or rather square) gets clipped to the boundaries (path) of the blue triangle, and this is done by making the rectangle a child of the triangle in the layers list. This is called "layer clipping".
In (B.) the rectangle acts as a mask applied to the triangle, and this is done by dragging the thumbnail of the rectangle directly onto the thumbnail of the triangle in the layers list. (Note the differences between the blue highlight bars appearing while the list entries are dragged to their respective positions.) Now it is not the rectangle that gets clipped to the boundaries of the triangle, but the rectangle clips (or cuts) parts of the triangle. This is called "creating a vector mask".
Hope you get the difference …
Kind regards, Alex