This is a free service and I love it. You sign up and can then start to build your own personal radio stations. If you don't want to take the two seconds it takes to do that, you can listen to any of the preset stations but that isn't as fun!
I currently have two stations. One I made during the holidays features just Christmas music by my favorite artists. The other station I named "My Favorites" (original, I know) and all I did was list some of my favorite artists and songs. From there, they grab music from their huge database and play the music I love in addition to songs that I might not be familiar with but are similar to my tastes.
When it plays music, you have the option of giving the songs a thumbs up or down. If you give a song a thumbs down, it won't play it anymore on that station. It is one of my favorite features. I have found some great new artists because of this site and I love how I can just let it play in the background while I'm working. It is a nice break from my iTunes when I'm in the mood for discovering some new music.
On my Pandora station right now: Sweet Child O' Mine by Guns N' Roses*. You know you love it, too.
*I was just talking with a friend today about how I hope Adam on American Idol sings this song.