Peer to Peer (P2P) is a means of file sharing data across the Internet by creating a network of remote computers that connect to a central point for indexing. The computer at this central point is the name of the P2P network you are joining. (Ex. Napster) However, when transferring files you do NOT connect to the central server, but you connect directly to the computer that has the data you wish to download. When you do that, your computer's IP address is recorded by the host system. You can kind of picture it in your mind as a hose that connects between your computers and the one that has the data you want. The logged IP address of the downloading computer is what the complainant uses to identify the computer that downloaded the copyrighted material - it's how they prove YOU were the one that copied the song / movie /computer program / whatever. To tie a specific IP address to a specific computer, the complainant subpoenas the Internet Service Provider (ISP) that 'owns' that IP address for the information about what account was assigned that IP address on the date and time the download occurred. ISP's maintain that data as a part of running their business - it's how they know to bill you and not someone else!
P2P uses the Internet infrastructure (TCP/IP communications) but is not really a 'part' of the Internet - the 'net is just how you use it. In order to use P2P you download a program from the P2P network that you wish to join, and install it on your computer. This creates an account for you, and opens a file share on your system where you can put data for anyone else in the world to download. Once you have done that you can then download anything that anyone else has out there. Of course you have to provide some details about the data you have made available describing its nature etc., as does everyone else that uses the network.
I'm sure that you have heard of Napster and Kazaa - both are big MP3 format song sharing P2P setups. Probably the other big P2P network is Limewire. The music and motion picture industry has been huge on suing people making the downloads for copyrighted stuff. You can actually download entire first run motion pictures and full versions of commercial software. That being the case - it's always prudent (if you are going to use anything from a P2P network) to be sure that you really aren't downloading a malicious virus infested piece of something!
None of these services are free of porn or other less than wholesome data! There are many data files out there that kids just don't need access to. OK - I know we don't burn books in this country and I support that every day in my work - free speech, press, assembly and all the other rights guaranteed by the Constitution - those are just plain good things! But my kids don't need to be reading the Underground Cookbook or Karma Sutra yet! They definitely don't need access to pornographic images and moves, not to mention child pornography. The media and general public just haven't picked up on what P2P networks have to offer - yet!
That doesn't mean that everything on P2P networks is bad! It's a great resource for sharing programs that YOU have written and have the authority to share - not to mention music YOU have written and recorded, photos that YOU have taken, stories that YOU have written, movies that YOU have made, etc. But the motivating thought here is the word YOU - it should be stuff that you have the authority by virtue of creation (or in some other fashion) - to distribute as you see fit. This document is an example of that - I wrote it - and I can share it for free if I should so choose - and I do!
Options for protecting your self from home include running firewalls on your home computers that block the ports that the P2P networks use, and not letting the kids have administrator rights to install programs (that prevents installing the P2P front end program) or make changes to things like the firewall.
Napster, Kazaa, Limewire and the rest of the P2P groups out there all require that you open file shares on your hard disk. Anytime you open a file share on your system and then put the system on the 'net for other people to access you are running a security problem in your home. From time to time 'the computer underground' breaks something in Napster, Kazaa, Limewire and the rest, and then obtain full user access to these systems without the owners ever even knowing. I run a web site from my basement and know how hard it is to secure systems connected to the 'net - opening P2P systems is just plain a bad idea from my 'parental perspective'.