The BitTorrent protocol works by using pieces as trading units. It breaks the file-set described by a .torrent file into a virtual set of pieces (usually between 64KB-4MB in size) which are then exchanged between peers.
When you create a task out of a torrent file by adding it into your BitTorrent client, as soon as you start receiving a few pieces, your client starts uploading them towards other peers it’s downloading from (which do not have at least one of the pieces you posses). If it didn’t upload as well, at the same time with downloading, your client would be locked out by the rest of the swarm and your downloading speed would quickly drop to 0.
Once you have the whole contents of the torrent file-set your client transforms into a seeder for that torrent (a.k.a. it altruistically offers the pieces it possesses without demanding any in exchange from other clients). It’s as simple as leaving you task running after it finishes downloading.
The main issue is that the great majority of people have asymmetric Internet connections (much higher download speed than upload speed) therefore the download of a torrent finishes way before you uploaded as much data as you have downloaded.
So, if everybody would stop their tasks immediately after finishing the download, then all torrents on the net would be pretty quickly dead. Seeding is a **paramount **activity in the BitTorrent Network; it’s what keeps it alive and what enables you, me and everyone else to find what we’re looking for on the network.
It’s due to the good faith of others, that we’re able to find torrents still being alive even years after they were uploaded and started by their original uploader.
That’s why the Share Ratio statistical parameter was created and introduced in all clients’ interfaces, so that one could watch at any time, how much has been uploaded for a certain torrent.
The share ratio is calculated by dividing the uploaded data by the downloaded data size. Therefore a share ratio of 1 means you just have uploaded as much as downloaded.
That’s a “minimum minimorum” to upload for any given torrent, otherwise you’ll be a leech. In fact, one should upload some more (1.5-2 is an usually acceptable ratio), or much more, especially if that one particular torrent doesn’t have many seeds/has a rare or old content on the net (this way you can make sure that others will be able to fell the same joy you felt, when you found that rare/old content in a torrent and were thankful that somebody was still seeding it).
For more info on the subject read also this BitComet Wiki topics: BitTorrent, the intro part of this topic and also this FAQ topic.