It's flashcards++: They can have audio attached, for example, and multiple "sides" to the flashcard, but the most ++ part of it is the integrated spaced repetition algorithm(s) that automagically adapt the next time a card is shown based upon its difficulty to improve long-term recall without having to look at all of the cards super often. This is termed spaced repetition.

It's really good. It's still flash cards and best for things where you just have to memorize something (I use it for mandarin characters, for example, for which it's really excellent). But it's great at what it does.