I've come to think of tracks as falling into a few categories, describing how I'm likely to use them. (Some tracks incorporate elements of more than one type, of course.) Some examples:
Openers: These tracks have strong opening moments, immediately setting the scene or inspiring a reaction from the players. Sometimes I only use that beginning bit of the cue, but some of the best openers lead into tracks that are good all the way through for ambience. (A short track that's just a quick dose of dramatic music is sometimes called a "stinger.") A good opener conjures an immediate vibe, whether it's a jolt of horror or a heroic rise, which can be a great response to player actions. Play that ominous opener when the characters discover the latest murder victim; play that rousing fanfare when it's time to roll initiative and things break out into heroic combat.
Repeaters: Repeaters can be long or short, suitable for quick-paced action or drawn-out atmospherics, but they're always good all the way through and for more than one play at a time. You want a good collection of atmospheric repeaters to play in the background of dialogue-rich or exploration encounters as well as a slew of great action and battle cues that you can play over and over without getting too tired of them. Just set your device to repeat and focus on the gameplay for a while.
Repeaters are also valuable pacing mechanisms. Having the same track on repeat helps you recognize when you're letting something drag on. (Depending on the piece of music, the track's constant repetition might actually emphasize that things are dragging and spur players to action.)
Dramatics: Dramatic cues have musical twists, dips, or crescendos that wonderfully underline or announce some dramatic event like a critical hit or the death of a boss monster. These cues are tricky: They have great dramatic turns or narrative arcs within them that are tempting to use for key moments of play, like when the tide of battle turns or a climactic revelation emerges, but they require careful timing. To get the most theatrical effect out of a dramatic cue, you must time your narration just right, announcing the monster's dreaded attack or climactic defeat just as the music crescendos behind you. This takes practice and a comfortable familiarity with the piece of music you're playing.
If you know a track well enough to time your speech to it, just about anything can be a dramatic cue. When all you're doing is narrating - whether it's the somber funeral of a heroic king or the triumphant celebration of his successful rescue - dramatic cues can support your words with an emotional undercurrent. When you're in the midst of a hectic combat turn, though, you'll have to let some of the crescendos go by. Part of being a good Gamemaster DJ is remembering that the game comes first; don't sacrifice clarity or fun just to make some musical cue work for you. Don't expect your players to sit still, even for a minute, and listen to a piece of music you think says it all - roleplaying games are a conversation, and if the talking stops, so does the imagery.