A leech is a person who sucks data, resources or bandwidth away from a network or website in a way that’s considered ethically questionable or shows poor netiquette. This usually means placing a burden on the target without reciprocating something of equal value. Leeching has an additional meaning in the world of torrents, and again in USENET binary newsgroups where being a leech is perfectly acceptable. So what are the differences?
In the wireless world, a leech is someone who steals bandwidth by stationing himself outside a wireless café, for example, so he can use the service without patronizing the café. A neighbor who uses her neighbor’s network without permission is also considered a leech.
Another type of leeching occurs when a website links to an object located on an unaffiliated site. For example, Bob builds a website and links to an image on Jean’s site. When someone visits Bob’s page, it looks like the image is located there, but actually Bob’s site requests the image from Jean’s site with every visitor to his page. Bob is stealing bandwidth from Jean, as the server hosting her site uses her bandwidth allotment to serve the picture to Bob’s site. Bob is a leech.
In the world of torrents everyone who downloads a file is termed a leech until he or she receives the entire file and begins seeding it back to others. In this sense being a leech is a function of the network. However, successful torrent networks are based on equal sharing, so anyone who takes more than they give over a period of time is considered a leech in the negative sense. Torrent software discourages leeching by throttling back bandwidth to those with low share ratios.
In USENET binary newsgroups where files get posted for download, leeching does not have a negative connotation because newsgroup protocol does not rely on equal sharing. People can download all the files they want without a responsibility to reciprocate. Hence, a USENET leech is not a bad thing, as evidenced by one of the most popular binary newsreaders, NewsLeecher.
Aside from USENET and the necessary function of torrent downloading, being a leech is a tag to avoid. Everyone benefits when people practice good netiquette, which is the reason the concept exists.