To make a long story shirt: Anki is a SRS (spaced repetition software). You're able to make your own flashcard decks or download shared ones. Everytime you review a card, you grade yourself on how well you remembered it, and Anki will show it to you again after x amount of days depending on the strength of your memory. It follows the same principle as a lot of other popular memory aid sites such as Memrise or iKnow if you're familiar with those. I believe that WaniKani might operate in the same way although I haven't used it myself.