| The basic form of a butterfly valve is a shaft with a circular disk attached. Opening and closing the valve is a matter of turning the shaft 90 degrees to move the disc parallel to the flow to open it or perpendicular to the flow so it blocks most or all of the flow through the valve to close it. The edge of the disc may make contact with a metal or elastomeric seat, a liner, a step, or not make contact at all. In the latter case, the valve is actually a damper such as you may have in the heating ducts in your house or office. It is possible to have square or rectangular discs as well for square or rectangular ducts.
In some butterfly valves, the shaft center is exactly where the valve center is. For high-performance valves, the shaft center is slightly off the valve center so that disc cams into the seat and is not in contact with the seat in the intermediate positions. This is called an 'offset shaft' type butterfly valve. Offset shaft butterfly valves can only be operated in a particular 90 degree quadrant.
Another characteristic, perhaps the reason why butterfly valves are called such, is that for most of these valves, the disc sticks out past the connecting ends into the pipe when the valve is fully open. The valves can have wafer or lugged ends where the valve is sandwiched between flanges, but double flanged and weld end butterfly valves are common items.
Butterfly valves are less expensive than ball, gate or globe valves starting from around 2 or 3 inch pipe size. They have a smaller flow capacity than ball valves of the same pipe size. Like ball valves, butterfly valves can be manual, actuated or control valves. They can be made from a variety of materials such as plastic, cast iron, cast steels, forgings or noble materials such as Titanium and Hastelloy.
Butterfly valves are also susceptible to a phenomena called flutter which is caused by hydrodynamic forces exerted by a fast flowing media on the disc causing it to oscillate. Because of this, they have a limited range of application as control valves. Also because of this, electric actuators used on butterfly valves must have a motor brake. | 
