Go to Pandora.com, click "Create a New Station" and type in the name of a band you like. Immediately, music begins streaming—a song from your artist, followed by songs that should appeal to you. "Should" is the operative word here: the service is streaming songs that Pandora's creators at the Music Genome Project have determined as having similar musical characteristics.
Each song in the MGP's database—collected work from over 10,000 artists, indie and big time—has 400 isolated "genes." The MGP's music experts listen to the songs and catalog them every which way, evaluating things like instrumentation, influences, rhythms, harmonies, tonality, and a bunch of technical musical stuff that I don't entirely get. Their technique isn't untried—they've been building systems with this sort of recommendation system for clients like Barnesandnoble.com for years. Because every song is evaluated by a human rather than a machine using a simple algorithm ("people who buy A often also buy B"), and because the humans involved are actually trained experts with real-life musical composition experience, the recommendations are more appropriately targeted.
At first, no matter what artist you enter, you're going to hear songs you like and songs you don't. After all, the system is looking for songs that have any of 400 possible similarities. Click "Guide Us" and give songs a thumbs up or thumbs down, and the system starts drawing further conclusions. You can skip the stuff you hate, though due to the way the digital rights management controls the streaming audio, there's a limit to how many songs you can skip in a session. As you work on it, you'll start steering your way towards music you've never heard of—but somehow enjoy. That is Pandora's goal.
Each artist you enter becomes the theme of a new streaming channel, so your Public Enemy moods and your They Might Be Giants moods will never need to overlap. Here's a tip, though: Don't put your absolute favorite band in there. At least, not at first. I love Dire Straits. It was a little unnerving to be told what musicians share Dire Straits' musical characteristics. I can handle Robert Cray and Nils Lofgren, but Billy Joel and Toad the Wet Sprocket? Skip!
The best station I made was centered around a cool folk singer named Greg Brown. I love the guy's work, but he doesn't have a monopoly on gravelly harmonies and ramblin' acoustic guitar. I knew that going in, and was happy to get a stream of his fellow folksters, including some exceptional unknowns, as well as giants like Bob Dylan and Cat Stevens.
The thing is, you just go in and do it without signing up for anything. They don't even ask for your name, let alone your credit card, so give it a try. If you get through 10 hours of it and still haven't felt out at least one channel you like, just walk away.