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I believe that you are referring to the
old-fashioned push-the-handle-down-and-
water-comes-out-of-the-spout lift pump.
1. Pressing the pump handle down lifts a piston
surrounded by a wet leather gasket that closely fits
the vertical water pipe of the pump.
2. As the piston rises, the air pressure below the piston
is reduced and water below the piston is forced upward
in the pipe by the greater external air pressure surrounding it.
3.Raising the pump handle then lowers the piston
into the water and some of this
water goes through a one-way valve in the piston
as it is lowered and cannot return through
the one-way valve.
4. Lowering the pump handle again , raises the piston.
As the water above the piston spills out of the pump spout,
water below the piston again rises due to the lowered
air pressure.
Further lifting an lowering the pump handle continues
to maintain an outflow
Herb Gottlieb from New York City
(Where water comes out of our faucets at home without
the benefit of human inervenion)