In a nutshell: It's a place for people to upload twelve-song MP3 mixes and share them with other people. That's it. Signing up is free, uploading is free and the chances to be exposed to new music are limitless. (Plus, nerd humor abounds: This one spoofing the hipster-herd mentality/blog-hype cycle was linked everywhere, but still funny.)
I of course made a Muxtape, comprised of new songs I'm digging so far this year. Click that link there to hear it.