Peer-to-Peer

Peer-to-Peer (P2P) is a communication model which allows users to communicate together using a decentralized network. Peers pool together their resources and make them available to other uses without the need for a middleman relaying data.

The most popular P2P technology is the BitTorrent Protocol which is commonly used to transfer large files in pieces between users with the use of a tracker.

The first popular P2P file sharing system was Napster, which was released in 1999 and shutdown not long after due to high copyright rates.

Misconceptions
If I have a BitTorrent client installed on my machine I could get in trouble P2P technology is only useful for piracy
 * This isn't true, there is nothing illegal about having a torrenting client installed on a computer. All BitTorrent clients do is allow efficient transferring of large files, what files you choose to transfer is what will determine if you're breaking a copyright law.
 * There are plenty of examples of the BitTorrent protocol being used for legitimate purposes and being extremely useful. See "Examples".

Examples
There are many companies and situations utilizing P2P communication for legal means, and there has been great success in doing so.
 * Windows 10 updates
 * Distribution of Linux distros
 * Sony releasing the movie The Interview after receiving threats