You can buy music either from the Android Market on your phone/tablet, or from the new desktop store. Google's partnered with the big 3 record labels in addition to tons of indie labels, giving them access to about 13 million songs (8 million of which are currently live on the store). Individual tracks run about the same as iTunes: many tracks are $0.99, while more popular artists might run you $1.29 a track. A typical album is either $9.49 or $10.99, again, depending on popularity. All tracks have 90 second previews and are downloadable in 320kbps mp3 format.
Check out http://music.google.com/about