ok - I just did the research =D
A second cousin is the child of your cousin.
From there, a third cousin would be the child of the child of your cousin.
So for example, you're cousins name is James. He has a son named Todd (Todd is then your second cousin) Todd has a daughter named Lucy (Lucy is then your third cousin), Lucy has a daughter named Lily (Lily is your fourth cousin), and on and on the generation line. =D
And according to the very confusing wikipedia (well, normally wikipedia isn't confusing, but today it was. lol) your moms cousins son would be your moms "second cousin" and to you, that would be your "second cousin once removed". I think. Family tree stuff bugs my brains. lol!