Good question. Spaced Repetition Learning is any method based on scheduling when you will next review a bit of knowledge. For my purposes, however, I'm going to confine my discussion of SRLs to those that: a) deal only with long-term memory (thus eliminating methods such as Pimsleur where the intervals for review are scheduled in seconds, not days,) and b) increase the review interval after each successful review.
My "favorite" SRL system is called SuperMemo, created by Piotr Wozniak (no relation to Apple's Woz). SuperMemo uses a flashcard metaphor for the learning process, but in its latest iteration it is capable of sophisticated multimedia content. Basically, whenever you learn a card containing a piece of knowledge, the computer records the date of learning and schedules the next review one or two days out. Then you are on to the next piece of knowledge until you are done learning new material for the day. The next time you power up the program, it checks for what cards you are scheduled to review for that day and proceeds to test your ability to recall the information. You grade yourself, and depending on how well you remembered the card's info, it will again schedule the next repetition for a date before it predicts you will forget what you reviewed. For a piece of knowledge of average difficulty, the schedule may go like this: 1 day, 6 days, 14 days, 35 days, and so on. If you find that your recall comes very easily, the review intervals may be even longer; more difficult recalls result in shorter intervals, and the occasional lapse will trigger a reset of the schedule because it is now necessary to relearn that bit of knowledge.
SuperMemo boasts an increase of 10-50 times the rate of learning. Don't think, however, that this is going to help you pass your mid-term exams next week (or was it last week?) SRLs are designed to enhance long-term memory, so if you want to remember something a month after the test, this is for you. The real boost to your learning comes from the ability to process new information because your review cycles are made as long as possible without actually forgetting what you learned.
One other benefit I have found relates to David Allen's Getting Things Done (GTD) methodology. Allen says that a lot of our mental stress comes from our mind's effort to hold on to things we are afraid we will forget, all the "open loops" in our lives. I know that if I put something I want to learn in SuperMemo, I will review it at some future date. That way, even if I have forgotten it, I at least can relearn it. This takes a lot of worry about whether I'll remember something critical out of the equation for my mental stress. And if I have some idea that I want to develop later, I just put it in a queue of items that will eventually work through this process.
I know that there is a lot more information available, so I hope I haven't upset any SR gurus out there by omitting your favorite aspect of the method, or your particular flavor of SRL. After all, that's what the comment section is for!