Briefly, what you do is plug in the name of a group (or even a song). A "radio station" is set up that includes other songs by the group, and music from artists whose music is within the same "genre." For example, I set up a Squirrel Nut Zippers radio station. Besides music from the SNZ, I get songs from Madeleine Peyroux, Big Bad Voodoo Daddy, and Ella Fitzgerald.
I can minimize the screen and listen to the music all day long! And, the best part--it's free! If I mind the occasional ad for Polish vodka (or other sponsor) that may appear on the side of the screen, I can pony up a few bucks and pay for ad-free music, but since I'm listening and not really reading the screen, the ads don't bother me.
There are other features on the site like a blog, information about the groups or the albums that are being played, etc. I encourage you to check it out!
Speaking of freebies, the CDs at the library are always free for the borrowing! If I weren't familiar with Madeleine Peyroux, and had heard her on my "Squirrel Nut Zippers station" and liked her music, I could have come to the library and found that we had 3 of her CDs in our collection:
Careless Love. [CD FEMALE VOCALIST PEY]
Dreamland. [CD FEMALE VOCALIST PEY]
Half the Perfect World. [CD JAZZ PEY]
Of course we don't have everything, but you might be surprised by what we do have in our collection. (Alas, no Squirrel Nut Zippers--yet!)