Something used to clean
. Usually a clogged drain
, although some drain cleaners
The two main kinds of drain cleaners are mechanical, chemical, and biological. Wait. The three kinds of drain cleaners are.. nevermind.
Mechanical drain cleaners are things like plungers (pressure loosens clogs) or snakes (you physically rip apart or break through the clog). They are very safe on the environment, but are a bear to use, and may not work.
Chemical drain cleaners are almost always an extremely strong base. Like lye. Some of these have other ingredients designed to get them in the pipes faster if your sink or tub is full of water, or to cling to the clog longer, but basically the active ingredient dissolved the fat, soap, and hair that most clogs are made of. They are usually very effective, but dangerous both in use and in possible long-term environmental effects.
Biological drain cleaners really aren't designed for emergency use, but are god for preventative use. They consist of enzymes or bacteria that actually eat the fat and scum in a drain clog as well as on the [walls of pipes. Used regularly, they can keep your pipes from clogging almost at all.