Marc Pesce descreve muito bem o conceito de P2P e Bittorrent:
BitTorrent is a computer protocol (a language computers use when communicating with each other) which allows computers to freely and efficiently share information with one another. This free-for-all of sharing is often called peer-to-peer or P2P, and it has become one of the most popular activities on the Internet.
If you're just one person with one recording of one show, and it's a popular show, your computer's internet connection is going to get swamped with requests for the show; eventually your computer will crash or you'll take the show off the Internet, just so you can read your email. And in the early days of peer-to-peer, that's how it was. Someone would find a computer with a copy of the song they wanted to listen to, connect to that computer, and download the data. It worked, but anything that got very popular was likely to disappear almost immediately. Popularity was a problem in first-generation peer-to-peer networks.
In November 2002, an unemployed programmer named Bram Cohen decided there had to be a better way, so he spent a few weeks writing an improved version of the protocols used to create peer-to-peer networks, and came up with BitTorrent. BitTorrent is a radical advance over the peer-to-peer systems which preceded it. Cohen realized that popularity is a good thing, and designed BitTorrent to take advantage of it. When a file (movie, music, computer program, it's all just bits) is published on BitTorrent, everyone who wants the file is required to share what they have with everyone else. As you're downloading the file, those parts you've already downloaded are available to other people looking to download the file. This means that you're not just "leeching" the file, taking without giving back; you're also sharing the file with anyone else who wants it. As more people download the file, they offer up what they've downloaded, and so on. As this process rolls on, there are always more and more computers to download the file from. If a file gets very popular, you might be getting bits of it from hundreds of different computers, all over the Internet - simultaneously. This is a very important point, because it means that as BitTorrent files grow in popularity, they become progressively faster to download. Popularity isn't a scourge in BitTorrent - it's a blessing.