Ball Valves are used to regulate water flow through a pipe. They allow for complete flow when fully open.
Free Flow Check Valves are used to provide flow through a pipe in one direction only. When installed between an external filter and the pond, it keeps the dirty water in the filter from flowing back into the pond if power is lost. They are also used to keep an external pump primed if power is lost.
Spring type check valves should never be used in a pond installation.
Gate Valves, referred to as knife valves by some, are easier to use than a ball valve for fully opened or closed operation. They are not used for regulating flow.