You've probably got that tech-savvy friend who gets movies on his computer weeks before they officially come out on DVD.
Or maybe he's acquiring books and albums put out by independent artists who actively encourage people to download their output for free.
Odds are that this is being done via BitTorrent, a highly effective way to transfer files over the Internet.
Here's what you need to know.

BitTorrent

If the Internet is a highway, then there are many different ways of transporting files down that highway.
FTP (file transfer protocol) is a way that content creators put media online for people to access.
HTTP (hypertext transfer protocol) is what transports files to your computer so you can read this very webpage.
BitTorrent is simply one more way of moving files around the Internet. Before delving into how it works, let's learn some BitTorrent-specific vocabulary.

Torrent file

A torrent file is a piece of data that you download and feed to a BitTorrent client. It tells you what data is being downloaded and it tells the client where to go to find the data.

Client

A BitTorrent client is a piece of software that accepts a torrent file and begins downloading the data associated with it. The same way that you use a web browser like Firefox or Chrome to browse the web, you need a BitTorrent client for your computer to make sense of a BitTorrent file.
There are numerous clients out there, but our favorite is Transmission.

Seeders and leechers

A seeder is a person who is uploading a completed version of a file to whoever wants it. "Leecher" usually refers to a person downloading a torrent and isn't yet seeding.

How it works

Imagine you want to acquire a 700 MB movie file. To download it from a single source could take quite a while, but BitTorrent is smart enough to seek out multiple people who have the same movie file and download it in chunks from several sources at once.
This means you get the file much more quickly that you normally would, and it's what makes BitTorrent such a popular means of piracy.
This brings us to the next part...

Legality

The same way that you can legally own a gun for recreational target practice and hunting, you could also use it to rob a bank. BitTorrent is a perfectly legitimate technology, it's simply a matter of what you use it for that makes it legal or illegal.
That tech-savvy friend who's downloading movies weeks before they come out on DVD? He's probably breaking the law.
Your friend who's using BitTorrent to download a new indie movie with the director's blessing? He's got the right idea.

Where to find torrents

With some clever Googling, you can find almost anything you want, but we're going to encourage you to download legal torrents that can be found on a number of services, such as ClearBits.
Check out this excellent collection of legal torrent services on GigaOM.

Rounding it up

BitTorrent is awesome. So many different types of media fly under the radar and never find their audience, but BitTorrent is a way of decreasing the distance between producers and consumers.
So what are you waiting for? Download a client and start torrenting!