Spaced repetition is a technique for efficient memorization & practice of skills where instead of doing a lot of work quickly, each item’s practices are automatically spread out over time, with increasing durations as one learns the item. I review what this technique is useful for, some of the large research literature on it and the testing effect (up to ~2013, primarily), the available software tools and use patterns, and miscellaneous ideas & observations on it.
Source: Spaced repetition – Gwern.net