According a malware analyst that goes by the handle Kafeine, the ransomware shows a message with the logos of the US Department of Justice, Homeland Security, and the FBI, and includes information such as the user's IP address, host name, and the URL of a porn website (not necessarily illegal) that the user has recently visited.
It does so by checking the browser's history, comparing the sites it finds there with a remote list, and if it discovers a matching website URL, it displays it in the warning message (click on the screenshot to enlarge it):
If it doesn't find a matching URL, it simply uses a random one.
In this particular case, the criminals are asking for $300 for the problem to go away and for the computer to be unblocked. Needless to say, users are advised not to pay the ransom but to search for a solution to the problem online (via another computer, of course).