A browser fingerprint comprises information obtained about a browser during normal use. Fingerprints can fully or partially identify individual users or devices even when cookies are turned off.
Tor works to anonymize browser data as much as possible without making it too obvious that the brower is running Tor. The Tor browser bundle aims to provide 'lowest common denominator' data in its fingerprint in order to fit in the most web traffic.
To learn more check out the Electonic Freedom Foundation's Panopticlick fingerprinting tool.
