Anki is a program designed to help you remember facts (such as words and phrases in a foreign language) as easily, quickly and efficiently as possible. To do this, it tracks how well you remember each fact, and uses that information to optimally schedule review times. With a minimal amount of effort, you can greatly increase the amount of material you remember, making study more productive, and more fun.
Anki is based on a theory called spaced repetition. In simple terms, it means that each time you review some material, you should wait longer than last time before reviewing it again. This maximizes the time spent studying difficult material and minimizes the time spent reviewing things you already know. The concept is simple, but the vast majority of memory trainers and flashcard programs out there either avoid the concept all together, or implement inflexible and suboptimal methods that were originally designed for pen and paper.
Ankis primary target is people studying Japanese, and Japanese native speakers studying English. However, it can be used to remember anything at all, and there are some users who are studying Chinese and possibly other languages with it.
