It allows you to specify the music you like and based on the Genome of the music specified it will play other songs that it thinks you will like.
The great thing about this service is that it does not require any download, accept for the FLASH plug-in to be installed on your favorite browser.
Why do I think Pandora is great? Well we have to begin by talking about the problem with many of the current online music stores (iTunes Music Store, Connect, etc.) and similarly the brick and mortar stores (HMV, Best Buy, SAM's, etc.). The fact that it is difficult to find what you (the individual) like. More importantly, what I may like based on my music preference. Pandora solves this problem by using the Genome of the music you specify to seek out other songs that you may like.
This is similar to the technology applied with Amazon.Com and other online retail, but this is done without other members' input.
Pandora's Genome library is built by professional musicians, each with at least 3 years of professional music experience, performing classification of a song.
Each song takes about 3 hours to classify and each musician participating had gone through extensive training on Music Genome.
You can check out My Radio Station at Pandora.