The term originated in the bulletin board system era, when it referred to users that would download files and upload nothing in return.
Depending on context, leeching does not necessarily refer to illegal use of computer resources, but often instead to greedy use according to etiquette: to wit, using too much of what is freely given without contributing a reasonable amount back to the community that provides it.
The word is also used without any pejorative connotations,[1][2] simply meaning to download large sets of information: for example the Usenet newsreader NewsLeecher.
Reaching an upload/download ratio of 1:1 (meaning that the user has uploaded as much as they downloaded) in a BitTorrent client is considered a minimum in the etiquette of that network.
In the terminology of these BitTorrent sites, a leech becomes a seeder (a provider of the file) when they have finished downloading and continue to run the client.