Put each term in turn into Google and select IMAGES
Gate Valve returns 347,000 pictures (a gate is lowered to control the flow)
Isolation Valve 95,800 (isolates part of a system)
Stop Cock 7892,000 (usually isloates a whole system)
Stop Tap 4,390,000 (more socially acceptable version of above)
Peccavi, April 2009