I often see people abbreviate faculty teaching loads ("course loads") as 2:2 or 3:2:3 or 2:1. Sometimes a note such as "3:3 with 2 preps" is added.
What do these abbreviations mean?
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]
To answer the part of the question about preps: It's easier to teach two sections of the same course than it is to teach two entirely different courses. A 3:3 load with two preps means that you teach 3 sections per semester but two sections are of the same course. For example, you might teach two sections of Calculus I and one section of ordinary differential equations for a 3 course load with two preps.
In the US, most academic courses are 3 credit hours, nominally meeting for 3 hours per week (often 3 sessions of 50 minutes each on Monday, Wednesday, and Friday, or two sessions of 75 minutes each on Tuesday and Thursday.) However, you'll find some courses that are two credit hours or four credit hours (or even five credit hours in some science courses with labs.) In assigning teaching load it's common to just count all of these as one course even though four credit hour courses take more time than 3 credit hour courses.