The idea is that you take a standard test administered much like the SAT's. CLEP offers 33 exams in five subject areas, covering material taught in courses that you may generally take in your first two years of college. Most CLEP exams are designed to correspond to one-semester courses, although some correspond to full-year or two-year courses.
Their TV advertising program expressed the concept rather well. The ad showed Abraham Lincoln being interviewed for a job. The interviewer asked him about his education. Abe responded that he had done a lot of reading on his own and had apprenticed at a law firm. The interviewer shook his head and said that they required a diploma. The test is to recognize knowledge you might have gotten some other way. How about the excellent, and Free, online classes you can take, which poor Abe couldn't use, but you can?
Exams are approximately 90 minutes long, with the exception of College Composition, which is 120 minutes. Exams contain mainly multiple-choice questions. College Composition and a few other exams contain other types of questions and essays. They cost just under $100 per exam, so much lower than a college class.
If you get the required score, based on the requirements of the college you plan on attending, you get two benefits: First, the school gives you credit for the comparable class just as if you had taken it, at no cost. Second, you don't have to take these classes and can jump ahead to the next "real" class in the sequence.
I'll use myself as an example. (They have changed the names of the tests since I took them, and have added more, so you'll have to adjust the terminology, but you'll get the idea). I passed the tests that correspond to English Composition I and II. On my transcript, these showed as English Composition I and II with a notation about them being from the CLEP test. I could start my freshman year with a more advanced English class at the sophomore level. I also passed the College Algebra test, allowing me to skip the 1-semester basic class, so I could start with Algebra II. They have changed the history and social sciences tests a lot since I took them, but I tested out of two semesters of classes there. I also passed the biology and a chemistry tests, although I can't imagine how. Today they offer tests not available then, which means I could have tested out of even more. Remember, I was just a solid B student. You don't have to be exceptional to pass the tests.
You should check out the program, see if the college you want to attend accepts them (there are 2900 that do), and consider buying the study guides ($10 each) to give you a leg up. With the high price of colleges, even community colleges, it is well worth your while to see if you can test out of a semester or even a whole year of college. Good luck!
P.S. I just checked a few more colleges on their policies. There are major differences, so check carefully. For example, Penn State doesn't allow credit for College Composition, but U of Mass does. Another school I checked didn't give actual credits for the classes but would use the test to meet a pre-req requirement. Yet another school gave credit for one of the three English Composition tests but not the other two, and the one they accept is not what you'd expect.