i dont know alot about torrents since i dont use them often but i can share some of what ive learned.
what is a torrent?
basically you d/l the .torrent file. this is a very same file typically only a few kb. its basically contains a record of the files included. basically its a link file.
so how do i get this .torrent?
you usually get the .torrent file from whats called a tracker. as the name implies a trackers is a site that tracks the file. who has the whole file/part of it/none of it ect.
so how do i actually get the data i want?
you load it into a bit torrent client. a few good clients are utorrent (windows also runs on linux with wine). azureas is another decent client with both windows and linux versions. after you load it it contacts the tracker and gets a list of all seeders and leechers as well as how much data each has. people that have the whole file are known as seeders (uploaders). everyone else is known as a leecher (downloaders) these only have part of the file. seeders are self explaintory they send you data. leechers are essentially downloaders however they also upload at the same time.
but how can leechers upload and download while not having the whole file?
a torrent is split up into pieces (im sure you remember this from emule). each piece is a small part of the file. once someone has a complete piece it notifies all other peers of that and will then be able to upload that piece to all other peers who need it.
how does nat play a part in all this?
basically if you are natted (ports not forwarded) you will get an unconnectable warning. dont be alarmed though. alot of people think that because of this they cant download. but this isnt totally true. you will still be able to connect to and download from anybody that is connectable but not those that are also unconnectable. since unconnectable users cant connect to other unconnectables there downloads may take longer because there is fewer people that they can download from but chances are they will still be able to max out there connection.
sorry if some of this doesnt make sense if your confused ill try to explain in more detail anything that confuses you. also if anyone else has anything to add please feel free. cheers