Just a quick correction (because it is a very common mistake). I think you are referring to "spaced learning" or "spacing", not "spaced repetition".
Every time you reinforce your memory, the amount of time it takes to forget it lengthens. Or, more properly, the slope of the curve that describes the probability of forgetting it decreases. That curve is known as the "forgetting curve". In spaced repetition, you are trying to time the repetition so that you still have a low percentage chance of forgetting.
"Spaced learning" or "spacing" is almost the opposite effect. What it says is that inserting a space in learning such that you forget something makes the material stronger when you subsequently relearn it. Although I am not aware of research that actually links spacing to the forgetting curve, you can probably imagine that if you learn something once, the forgetting curve is fairly steep (i.e., the probability of you forgetting it grows fairly quickly with time). With spacing, if you wait until you forget something and then relearn it, the forgetting curve is much shallower the second time (i.e. the probability of you forgetting it grows fairly slowly with time). In fact, it is apparently much shallower than if you had done spaced repetition (i.e. timed a reinforcement so that that allowed you to remember the item rather than forgetting it). Forgetting an relearning is rather an annoying and difficult way of learning, but some research suggest that it is dramatically more effective. This has been called "desired difficulty".
Also, there is a slightly nuanced difference between "blocking" and "interleaving". In interleaving, you fairly rapidly change topics and then loop back to the first again. So, if you have to learn 5 tasks, (A, B, C, D and E) you could spend 10 minutes on each and then loop back to the beginning. If you did that 6 times, you would have spent 1 hour on each task. Blocking would be spending 1 hour on each task, in blocks and then going back to it the next day. These are both different from the approach of training A until you learned it, then moving on to B, etc, etc.
In schools you can talk about "blocked" schedules (i.e. you try to fit in every subject every day -- not to be confused with the opposite of cramming a single subject into 3 or 4 periods, annoyingly also called a "blocked schedule"). Inside each subject they often don't interleave the material, though. There are some textbooks available that try to interleave the material within a subject. I have also read papers of some experiments where they actually tried to interleave all of the subjects (i.e., in each period you would have 10 minutes of each subject) with some success.
Anyway, I hope that helps. I spent a huge amount of time reading about this a few years ago, but have neglected it for about 3 years, so I hope I have not made any mistakes myself ;-)
No, I'm referring to spaced repetition, which is also often called spaced learning; the forgetting curve is why it works, but it is not otherwise relevant. The question about learning is how to distribute the reviews over time: all at once, or gradually over time with possibly expanding intervals. Similarly, with interleaving vs massed practice, the question is how to split up a fixed number of activities: should activity A be practiced for the maximum allowable amount of time, then B, then C and so on (analogous to massing on a single item), or should it be broken up with a few instances of A, then a few of B, and so on in a cycle until you've gone through all activities for the maximum allowable time? It turns out that for the same number of reviews or actions or exercises or time, spaced produces better recall than massed, and blocked/interleaved produces better skill learning than massed as well.
Every time you reinforce your memory, the amount of time it takes to forget it lengthens. Or, more properly, the slope of the curve that describes the probability of forgetting it decreases. That curve is known as the "forgetting curve". In spaced repetition, you are trying to time the repetition so that you still have a low percentage chance of forgetting.
"Spaced learning" or "spacing" is almost the opposite effect. What it says is that inserting a space in learning such that you forget something makes the material stronger when you subsequently relearn it. Although I am not aware of research that actually links spacing to the forgetting curve, you can probably imagine that if you learn something once, the forgetting curve is fairly steep (i.e., the probability of you forgetting it grows fairly quickly with time). With spacing, if you wait until you forget something and then relearn it, the forgetting curve is much shallower the second time (i.e. the probability of you forgetting it grows fairly slowly with time). In fact, it is apparently much shallower than if you had done spaced repetition (i.e. timed a reinforcement so that that allowed you to remember the item rather than forgetting it). Forgetting an relearning is rather an annoying and difficult way of learning, but some research suggest that it is dramatically more effective. This has been called "desired difficulty".
Also, there is a slightly nuanced difference between "blocking" and "interleaving". In interleaving, you fairly rapidly change topics and then loop back to the first again. So, if you have to learn 5 tasks, (A, B, C, D and E) you could spend 10 minutes on each and then loop back to the beginning. If you did that 6 times, you would have spent 1 hour on each task. Blocking would be spending 1 hour on each task, in blocks and then going back to it the next day. These are both different from the approach of training A until you learned it, then moving on to B, etc, etc.
In schools you can talk about "blocked" schedules (i.e. you try to fit in every subject every day -- not to be confused with the opposite of cramming a single subject into 3 or 4 periods, annoyingly also called a "blocked schedule"). Inside each subject they often don't interleave the material, though. There are some textbooks available that try to interleave the material within a subject. I have also read papers of some experiments where they actually tried to interleave all of the subjects (i.e., in each period you would have 10 minutes of each subject) with some success.
Anyway, I hope that helps. I spent a huge amount of time reading about this a few years ago, but have neglected it for about 3 years, so I hope I have not made any mistakes myself ;-)