What do the abbreviations 2:2, 2:1, or 3:2:3 mean in terms of teaching loads?
They refer to the number of classes you teach in each semester. ie 2:1 would mean two classes one semester and one the other.

Three numbers (like 3:2:3) refer to the number of classes in each quarter for schools on the quarter system (i.e., three terms per academic year). [Thanks to Mark Meckes]

Not all courses are the same size, although I suspect that the vast majority are the standard "3 credit hour" size. A minority but reasonable number of courses, though, are 4 credit hours (particularly in my area, math). A few courses are even 5 credit hours at some institutions. It depends on the field to a great extent. Nevertheless, the number of courses is still very important. Teaching a four-hour and a five-hour course for a 2-course load is less work, usually, than teaching three different 3-hour courses. [Thanks to Oswald Veblen]