NOTE: The level of dancers at ILHC is typically a lot higher than most events. Intermediate at ILHC would be considered advanced at most regional classes.
Intermediate:  You are becoming more comfortable with higher tempos and have a nice repertoire of moves on the social dance floor.  You have a good connection with your partner, can use dance tricks without interfering with your lead/follow and are adapting to different lead/follow techniques on the social dance floor.
Intermediate/Advanced: You have taken tons of classes, have a lot of moves, are beginning to feel comfortable in the competitive arena and are looking for that extra edge.  You are looking for a class that will work on the quality - not quantity - of your moves and your musicality, as well as refine your lead/follow technique.

Advanced:  You have been dancing long enough with a sufficient level and quality of dancing that you are actually hoping every class will be devoted to taking your swing-out, tearing it apart, and putting it back together again. "Tricks, schmicks, teach me how to dance."
Masters:  If you feel the need to ask if you should be in this level, the answer is likely not.