It keeps track of your history of failures, stats per-word, intelligently picks what word to give you next, and keeps bugging you about words until you've shown a history of getting it right (both times in a row most recently, and overall percentage).
You can create/change word lists, play again, etc.
I'm sure I'll add more later as I work on it more. But for now, I have a bunch of words to learn for a test on Tuesday. Clothing, colors, bunch of adjectives...
Time to stop hacking and start studying.