Chances are it's identified by Gracenote CDDB (compact disc database). Wired has a great story about Gracenote; how they started and where they are going. What I love most about Gracenote is the fact that they built their extensive CD database from the input of users like me. Back when Gracenote first started up, I would spend the time to enter information into Gracenote as accurately as possible so that other users wouldn't have to; I did this because other users were doing the same thing for me. Gracenote is currently used by the major online music stores, including Apple's iTunes Music Store; I love being able to put in very obscure or live CDs only to have Gracenote return the full disc information and track listing.
Gracenote has a new product called Music ID. From the Wired article:
. . . a file-recognition technology that analyzes the audio characteristics of a digital file like an MP3 or Windows Media Audio file. The service uses audio waveform technology to match music without any identifiable tags to Gracenote's database . . .
New this year, Gracenote's Mobile MusicID can identify snippets of songs through a cell phone. Music fans can dial a number and hold up their mobile phone near a radio, for instance, and Gracenote's service will send a message to the phone, identifying the tune being played . . .