Most oil wells use a mechanical pump with a two-way valve to extract oil from a well. A plunger is lowered into the well with an open valve, allowing it to sink below the level of the petroleum in the shaft. The valve is then closed, and the plunger brings up the oil when it rises.
In some cases, the natural pressure of the deposit is enough to bring oil to the surface when an oiler drills. These are called "gushers," and extraction is easy in these instances, at least at first. Once the pressure equalizes, the job becomes more difficult, and a sucker pump or other mechanism is required to extract oil from the well.
In many cases, drillers use additional means to increase the underground pressure to aid extraction. Injecting water, steam or chemicals into an oil deposit can force the oil to the surface or at least make it much easier to extract. Drillers can also inject carbon dioxide gas, repressurizing the well and forcing more petroleum to the surface. If multiple wells are drilled into a single deposit, a carefully managed pattern of injection and extraction helps pull as much of the petroleum as possible to the surface.