For those not aware, the UDID is an alphanumeric string that is unique to each Apple device. You can see your own by connecting your iPhone, iPod touch or iPad to iTunes and looking at the device information. Click on the Serial Number field (the value, not the caption) and you'll see the field change to Identifier and list a long string which is the UDID.
The UDID is currently used by mobile ad networks, gaming networks, analytics providers, developers and others.
UDIDs are of far more concern to people, as they are unique to a device, and cannot be changed. What developers and others will use in the future is still unclear. The IMEI, MEID or MAC address (for the networking portion) of a device could be used instead, but that could be said to present the same sort of privacy issues as a UDID.
report, Apple is moving forward quickly due to increased scrutiny from Congress and privacy advocates over the use of UDIDs.