Any time you see a valve with a wheel type handle it’s more than likely a gate valve. It’s called that because it has a gate that raises and lowers inside, as opposed to a ball that rotates, or a stopper with a gasket. Gate valves are used as main shut-offs and for isolating sections of pipe or manifolds.

The one thing every citizen of Earth should know is that gate valves need to be opened ALL THE WAY. That means turning that handle counter clockwise until it won’t turn anymore, lazy earthlings. The reason why is that moving water is a powerful eroder of things in its way. If the bottom of the gate is not raised up out of the way completely, ithe exposed lip will get worn away. When closed water will seep through the worn edge. Imagine a knight in shining armor charging out of the castle on his horse. Some castle door raising guy is slacking and doesn’t quite raise the gate all the way. The knights helmet nicks the bottom of the gate and takes out a chunk just big enough for mice to get through when the gate is lowered.

Exposed lip of gate, not opened all the way.

I don’t like gate valves used at main shut-offs. After many years of improper use, a system cant be properly winterized if the valve doesn’t shut off completely. There’s no point risking filling themainline back up with water as discussed in my test cock post. 

Gate valves are great for isolating electric valve manifolds. In that use it doesn’t matter if they leak, you just need the water off to open other valves and flush them.