A helipad can be anywhere. They are simply landing pads for helicopters. They are found on the roofs of skyscrapers, in front of hospitals, and in other places where helicopters may commonly land.
A heliport should be more like an airport. It should have fueling services, possibly a terminal building, ticketing services, air traffic control, night landing lights, and other amenities. Heliports are usually included in airport facilities, unless the location prevents planes from landing. I think it's rare to find a heliport that is not simply part of a larger airport facility.
I'd imagine there are many, many more helipads than heliports in the U.S.