Peer to peer (P2P) computing involves the direct connection of any two computers over the Internet, without the use of use of another server as a "middleman" to manage the interaction. The most popular use of P2P computing today is to share files over dedicated file-sharing network, typically to download music and video files.
On a traditional file downloading website, files are stored on a host computer for downloading to your PC. In a P2P file-sharing system, there is no host computer; instead, the users of the P2P system establish a network of personal computers over the Internet. You connect to the file sharing network and then search other users' computers for the files you want to download. When you find a match, the file is copied directly from that other user's computer to your PC.
Allowing another computer user to directly access your computer over a peer to peer network increases your security risk in a number of ways:
Spyware. One of the most common danger of peer to peer networks is the profusion of spyware, which often is piggybacked on the network's file sharing software. When you install the file sharing software, you may unknowingly install the spyware as well.
Unwanted file access. When you join a file sharing network, you agree to share certain files on your hard disk with other users of the network. Most file-sharing applications let you designate a single folder on your hard drive that can be shared with other users. If you subsequently place more sensitive personal files into this shared folder, those files are available for any user of the file sharing network to download.
Backdoor attacks. Any PC connected to a peer to peer network is a highly visible target for would-be attackers. Some attackers use peer to peer networks to distribute viruses that can later be used to access your system -- or use your PC as a remote-controlled "zombie." These viruses are often wrapped inside another file; for example, a virus might be disguised as an otherwise-innocuous MP3 file. When you download what you think is an MP3 file, you actually download the virus file instead.