You are not the only one who has wondered how gate valves work. They are commonly used, so it should be commonly known how they work right? Below you will find some interesting information on exactly how they do work.
A gate valve is a valve used to start and stop the flow of liquid.
How it Works
A gate valve is usually operated by a handle. Once the handle is turned it opens a "gate" so the flow of liquid can pass through freely, and the opposite. Take your sink faucet for example, turning it one way opens the gate, and allows water to pass through freely. Turn it the other way, and it closes the gate, restricting the water from flowing.
What They Aren't
Gate valves are most commonly used when minimal restriction is needed. They are designed to be fully open or closed, and are not recommended to regulate the flow of liquid. Using it for "throttling" causes strain on the gate and eventually will damage it.
Which One to Choose
Deciding which valve to use is a decision based on what you know about each one. Just keep in mind the above information when choosing one, and your decision making process should go just fine.