From wikipedia: A device fingerprint or machine fingerprint or browser fingerprint is information collected about a remote computing device for the purpose of identification. Fingerprints can fully or partially identify individual users or devices even when cookies are turned off.
Basic web browser configuration information has long been collected by web analytics services in an effort to accurately measure real human web traffic and discount various forms of click fraud. With the assistance of client-side scripting languages, collection of much more esoteric parameters is possible. Assimilation of such information into a single string comprises a device fingerprint.
Recently such fingerprints have proven useful in the detection and prevention of online identity theft and credit card fraud.