It refers to the act of giving yet providing nothing of substance in return, as it is named after the parasitic organism a leech.
can be applied practially to peer to peer file sharing protocols, or networks (Bittorrent, Kazza Fast Track
, Gnutella}, in which the leecher consumes bandwidth by downloading, yet has no will to upload or give back to a community.
Particularly in Bittorrent, leecher
s are not actually increasing their performance, but possibly are decreasing it.