I think that the full phrase (according to my Japanese for Dummies book, at least) it's "O genki desu ka?" which, for those who do not speak any Japanese, translates to something along the lines of, "How are you doing?" Though, according to, again, my Japanese for Dummies book, it is a serious inquiry about a person's health, and using it to greet a someone you just saw the day before is improper -- it's more of a phrase to use when you haven't seen them in a long time.
... I'm pretty sure, at least.