A juggle is a special kind of combo, involving the use of certain attacks (jugglers) on an opponent who is floating (which state is often caused by launching moves).
In 3D games, juggling is often the primary form of combo, whereas it is not highly common in 2D games, where air combos are more common (there are, of course, exceptions; Art of Fighting 3 and Street Fighter Alpha 3 allow some pretty intense juggles, for example).
In 2D games, there is such a thing as a juggle that can be repeated an indefinite number of times on occasion (see beanbag).
In 3D games, other than launching moves, there are moves that cause the opponent to collapse (during which state they are defenseless), and moves that cause the opponent to bound after tripping them up with a low move or something, among other approaches, all of which can lead into a juggle.
In general, juggles incur a lot of damage correction. Also, as the tracking changes, the necessary approach to successfully juggle changes, so knowledge is required.