It looks a lot like a flow controller which turns the pump on when a flow is detected (tap on ) and turns off when the flow stops.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

This would have been better in the plumbing forum,
but looks remarkably like this pressure switch. http://www.ecvv.com/product/1988317.html

If you look closely at your photo you can see the pressure gauge on the side of it.

This works on pressure and is usually coupled to a small pressure tank which stores a small amount of pressurized water.

When you turn the tap on, the water flows out of the tank, the pressure goes down, once the pressure reaches some set value (adjustable usually) the oump switches on an d pumps until you turn the water off, the tank pressurizes up again and the switch then turns the pump off when it reaches some defined pressure.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

Yes, that is a pressure switch which starts and stops the pump depending upon the loads. It takes care of a constant pressure, reduces peaks and can even act as a no-flow protection.