Spaced Repetition is a technique for long-term retention of learned material where instead of attempting to memorize by ‘cramming’, memorization can be done far more efficiently by instead spacing out each review, with increasing durations as one learns the item, with the scheduling done by software.
See Also: Scholarship & Learning
A good place to learn more about Spaced Repetition is Spaced Repetition for Efficient Learning by Gwern:
Spaced repetition is a centuries-old psychological technique for efficient memorization & practice of skills where instead of attempting to memorize by ‘cramming’, memorization can be done far more efficiently by instead spacing out each review, with increasing durations as one learns the item, with the scheduling done by software. Because of the greater efficiency of its slow but steady approach, spaced repetition can scale to memorizing hundreds of thousands of items (while crammed items are almost immediately forgotten) and is especially useful for foreign languages & medical studies.
The key insight for why spaced repetition should be effective is that you forget things approximately hyperbolically-- reviewing things very soon (as in cramming-style learning) is ineffective because you have not forgotten much yet when you come to a review. In comparison, Spaced Repetition allows you to renew your knowledge precisely as you're about to forget a given fact, giving the review the maximum return-on-investment possible and (over time) flattening the 'forgetting curve' so that the interval between successive reviews gets progressively larger for a given fact.
Obviously, it's not possible to remind yourself of something precisely when you're about to forget it. Enter Spaced Repetition Software (SRS)! By using the forgetting curve, SRS is able to plan when you need to review each item. You can either create decks yourself, or (for some topics) download from databases. Anki and Mnemnosyne are two popular free options, and SuperMemo is a subscription-based choice.
Criticisms of Spaced Repetition primarily revolve around the fact that, for it to be effective, knowledge has to be broken down into individual 'pieces' to go onto cards for testing. This is difficult or impossible for some types of knowledge, and may not promote an integrated view, where the structure or hierarchy of the knowledge is clear, as well as other methods. More can be found in the post A Vote Against Spaced Repetition.
Decks (links, or for Anki, the names of a deck in the Anki collection) relevant to LW.