pandora vs last.fm

i love last.fm with all my heart. i can always see what i'm listening to, and my recommendations, etc. pandora i just don't understand. i have so many questions! does it track the music you listen to or just the stations? does it have a separate application so you don't always have to have a browser window open? ah! i'm so confused...

sooo pandora or last.fm?
leave comments and decide!