The heart of Pandora is the Music Genome Project, a vast and ever growing taxonomy of musical information started back in 2000. They analyze songs according to nearly 400 attributes on aspects such as melody, harmony, instrumentation, rhythm, vocals, and lyrics.
Pandora then lets you stream and explore music. I started by giving Pandora a list of seed artists that Patti and I like, and Pandora took it from there - playing those artists, plus other artists and songs that are related via the Music Genome Project taxonomy. In just a couple of days Patti and I are already hooked - hooked enough to pay the $36 annual subscription to have Pandora advertising free.
There's a social networking and sharing aspect to Pandora too, and I've already bookmarked the Folk Holidays and Contemporary Folk shared music streams. And there's lots more to explore too. Like I said, I'm hooked - and excited.
The only complaint I might have so far is that Pandora's web UI is killing my "music PC", which is a 6 year old Pentium 4 with 1 GB of RAM running Windows 2000. Pandora is pegging the CPU on that old machine, and I can only assume it is due to its very nice, very slick, very modern Ajax web user interface, not the music streaming.
Oh, and if you are curious, the artists we seeded our Pandora stream with are Bruce Cockburn, David Bromberg, Diana Krall, Eliza Gilkyson, Lucy Kaplansky, Natalie Merchant, Ollabelle, Richard Shindell, Susan Werner, and Tish Hinojosa. I'm sure I'll add more, but those were no-brainers.