A UDID is Apple's unique identifier for each of its 'iDevices', and if known could be used to get a lot of personally identifiable information about the owner of each product.
The hackers released the data on pastebin here. In the interests of protecting the privacy of the users, they removed all said personally identifiable information from the data. This is kind of a shame in a way, as it would have been interesting to do an analysis of the geographic distribution of the devices which were (allegedly) being tracked, amongst other things. I suppose they released the data for more (allegedly) altruistic purposes - i.e. to let people find out if the FBI was tracking them, not to have the data analyzed.
The one useful column that was left was the device type. Surprisingly, the majority of devices were iPads. Of course, this could just be unique to the million and one records of the 12 million which the group chose to release.
iPhone: 345,384 (34.5%)
iPad: 589,720 (59%)
iPod touch: 63,724 (6.4%)
Undetermined: 1,173 (0.1%)
Forgive me Edward Tufte, for using a pie chart.