Not the area I usually hang out at, there are better techs than me at this board and there is a distinct difference between sounding like you know what you are talking about (those people often make my ignore list) and knowing what you are talking about.
That said, I have been using bittorrent for some years so I am acquanted with P2P, I am just no expert.
P2P stands for Peer-to-Peer protocol. One of the most common uses is bittorrent, I use Utorrent (it is small, easy, tested and free). In P2P file sharing you transfer files directly from your computer to other computers and vice-a-versa (hence the name). To keep up on file sharing and P2P news visit torrentfreak. The most infamous torrent site on the internet of all time is ThePirateBay.
Those are the highlights for specifics try wiki (the wiki knows all) or just use Google:
https://en.wikipedia.org/wiki/Peer-to-peer