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.
Besides text, it supports sounds, images and rendering TeX snippets in the cards. It can synchronize card decks to a server so that you can review the deck on other computers, a web interface or mobile devices, for which versions of Anki are also available. Complete card decks offered by other users can be downloaded the same way.
Anki is extensible with plugins which can be downloaded and installed from the menu. While Anki can be used for studying anything, plugins are available with special features designed to make studying Japanese and English easier: integrated dictionary lookups, missing kanji reports, and more.