Comcast has announced that it will start slowing down the traffic of its broadband users if they are using too much; "too much" generally means running P2P (peer to peer) filesharing applications like BitTorrent, which can run for hours or days while sending or receiving large files (like movies or music).
The strategy is reasonable, given that cable companies price their Internet services in part based on average use. Customers that far exceed those average use parameters slow things down for everyone else on that cable modem network segment, which often includes 100-200 neighbors. Cable modem bandwidth, like most wireless services, is shared among all connected users at an access point (wireless) or a cable network mode. Fortunately, not all users are doing something at the same time, but background applications like BitTorrent do run continuously for long periods of time.