angular momentum is a specific quantity that defines how much energy a mass has during circular/elliptic motion. If you are totally clueless perhaps you could look back on yr11 notes to make sure that you understand what lionear momentum is first.
Diffraction occurs EITHER when waves bend around a barrier,
eg you shine a torchlight on a keyhole, then you move to the other side of the door, you should see the lightwaves protruding from the keyhole. Projecting these waves onto a screen (eg a sheet of white paper), you should be able to see a pattern that describes the object of interference ie keyhole.
OR diffraction occurs when wavefronts interfere with each other. This is easier to explain diagramatically, but basically if you imagine waves as concentric circles with centres next to each other, interference occurs where the circumferences meet. The Michelson Morley experiment is a good example of this.
Hope you understand most of this, and if anyone has a better explanation of angular momentum I'd be interested as well