Stopcocks

A stopcock is a form of ball valve used to control the flow of a liquid or gas. Compared to the progressive control of a gate valve a stop cock's is coarse, confining their use principally to on-off applications. Stopcocks are used to grossly regulate the flow of tap water in residential and commercial services. 

For laboratory glassware, items sometimes have plug valves with conically-tapered inserts called stopcocks (sometimes referred to as "plugs" by laypeople) with different bores. The valve bodies are usually made of glass, while the plugs are made of glass or Teflon.