BitTorrent etiquette and the whole bittorrent principal is based on participants sharing as much as they upload. All BitTorrent clients will track how much of the file you upload against how much of the file you download. The relationship between these two amounts is called a ratio.
BitTorrent Seeders and Leechers
Those that download a file then disconnect from the BitTorrent tracker prior to uploading their fair share are considered to have Hit and Run.
The more users who hit and run, the more pressure place on those users who do the right thing with others users requiring to pick up the slack.
Public BitTorrent sites are notorious for large numbers of people who Hit and Run meanings the download speed often achieved from these sort of sites is very low in comparison to Membership sites who insist that you maintain a high ratio with their community.
Failure to maintain this ratio and adhere to good bittorrent etiquette will often result in a ban from the site.