File sharing networks, like Kazaa, are the source of much free music, movies, TV shows and litigation. The are also the source of classified documents. The Dutch Border Security service (Marechaussee) recently found secret documents of their current investigations had been found on Kazaa. No surprise there. Kazaa works by allowing any net users to automatically share files on their hard drive. You use the Kazaa software on your PC to connect, and search everyone elses hard drives for files you are looking for. Most people are looking for copyright violations (music and video files they would otherwise have to pay for.) The security problem is that when many people set up the Kazaa software, they make their entire hard drive available to the world, rather than just a few directories containing music or movie files they want to share. Thus other users, usually accidentally, discover files they didnt expect to find. Some less scrupulous people, knowing this sort of thing exists, go looking for files that contain financial or other useful information. 

The problem is actually worse, and older than, the file sharing networks. When someone creates a web site, and uses one of their computers as a server, it is not uncommon to make files available to the Internet that are not meant to be available to everyone. Again, unscrupulous people, with sufficient technical skills, go looking for these files. The objective is often just curiosity, but professionals are searching for data that can be turned into money, legally or otherwise. 

Cyberwarriors use this situation to constantly seek out government, commercial or military hard drives that are vulnerable. The only defense is to keep your local network off the Internet, which is what a lot of intelligence agencies and military organizations do. But you can never be a hundred percent secure. Someone is going to take work home with them, leave sensitive files on their hard drive, then start using Kazaa