Spaced repetition is an approach where material is reviewed at strategic time intervals so as to increase the strength of recall as much as possible. The general idea is to space out the intervals for reviewing the material with the gap between successive review intervals growing exponentially. The theoretical rationale for increasing the spacing between successive recalls is that the forgetting curve decays exponentially. Each attempt at recall increases the strength of the memory, therefore combating the forgetting curve.
Relation with mode of review
Spaced repetition is a general idea that can be combined with any mode of review. For instance, spaced repetition may be done using a quiz and recall strategy: a list of questions and answers is generated during the original study (or obtained from elsewhere) and the learner quizzes himself or herself using that list of questions after the appropriately spaced intervals. Spaced repetition may also use more active forms of recall (such as trying to reconstruct the whole knowledge base without any cues at all) or more passive forms of recall (such as reading through the learning material again).
The most typical use of spaced repetition is in conjunction with a quiz and recall strategy. Spaced repetition software such as Anki implements flashcards, that are ideally suited for this strategy. The reason for a quiz and recall strategy is that the attempt to actively recall material based on cues can do a better job both of testing one's memory and of increasing its strength. More active forms of recall run the risk of being too difficult for people who have not completely internalized the material, whereas more passive forms of recall run the risk of not strengthening the memory at all because there is no attempt at retrieval.