Very important devices, valves control the flow of liquids and gases in many situations. Gas appliances in homes use plug valves, the assembly in toilet tanks is a valve, and water spigots outside homes have valves.
Ball valves and plug valves perform the same function. Each kind controls the flow of liquids and gases either in a simple on/off function or as a multipath function.
Ball valves and plug valves are generally called "quarter turn" valves because they need their handle turned only 90 degrees to change on/off positions. This does not hold true for three- and four-way valves.
Ball valves can be full port or reduced port. Full port means the ball is oversized and allows for full flow; reduced port means the ball is roughly the same size as the inner diameter of the valve and restricts flow. Plug valves are mostly full port.
Ball valves tend to function reliably for years, even with long-term disuse. Some plug valves can eventually stick, becoming difficult to operate.
Ball valves do not offer the fine control of other valves for throttling purposes, but they can be used for some degree of throttling. In general, plug valves offer even less control for throttling.