As above indicates: it's a method for sending and receiving (sharing) files, usually large sized ones, through a particular service.
There are legit files to be shared, and then there are pirated copies of movies, and music that are made available. Copyright stuff is illegal to perform this method of sharing.
If you're thinking of doing this for 'free' movies or music, think again:
Bit torrent and other file sharing sites being used to get (free) music and videos can be total disaster for your Windows machine.
Because they use multiple 'peer contributors' to feed bits & parts of a requested file (music, movies, etc.) back to you, there can never be any kind of certainty about the status of the sender's file.
Whatever they have = you get: malevolent or not.
Once downloaded to your machine, all bets are off about it's safety to open, transfer, use or re-send; and size of a file has little bearing on what it may or may not contain; scanned or otherwise.
To get an idea of how prolific and devastating this can be just look at the number of 'Yahoo Answers> Security' requests for help about virus/spyware.
Most of these Yahoo users are younger & more likely to be using the trendy socializing sites and peer-2-peer services (Limewire, etc.). Hackers know this and target efforts to infect machines by these sources (aka 'vector').
Once infected, a long cleaning , endless aggravation & malware prevention steps will not guarantee immunity from a re-infection, because virus crafters constantly change the names, file properties, footprint & signature, source vectors or anything possible to propagate their garbage.
Use @ your own peril...
Torrents are small files that contain information that tells the computer where to download bigger files.
If you download uTorrent and get a torrent file called something like "Cool Movie", you can open the torrent with uTorrent and it will tell uTorrent to download "Cool Movie" from all the other people that have it.
The other people with the "cool movie" torrent who are giving it to others by uploading it are called seeders. The more seeders a torrent has, the more people uploading it to the people downloading it, which means that the download it faster.