Just as a garden hose nozzle is used to increase the outflowing dynamic pressure of the liquid, a pressure washer adds its own power to force high-dynamic pressure. Note that the pump does not draw more water from the pipe to which the washer is connected than that source can provide; therefore, the water supply must be adequate for a given machine to be connected to it: its flow must be equal or superior to that of the washer, so that the pump is never starved (which can damage it).
Several different types of nozzles are available, each useful for a particular application. Some nozzles cause the water jet to be ejected in a triangular plane, while others emit a pencil-thin jet of water, which spirals around rapidly. Most nozzles attach directly to the trigger gun.
Some pressure washers, in combination with a particular nozzle, allow detergent to be introduced into the water stream, assisting in the cleaning process. Two types of chemical injectors are available: a low-pressure injector that introduces the chemical after the water leaves the pump (a downstream injector) and a high-pressure injector that introduces the chemical before water enters the pump (an upstream injector).
Pressure washers are dangerous tools and should be operated with due regard to safety instructions. The water pressure near the nozzle is powerful enough to strip flesh from bone. Objects in the water supply can be ejected from the nozzle at great velocities. The cleaning process can propel objects from the surface being cleaned, also at great velocities. Pressure washers have a tendency to break up asphalt if aimed directly at it, due to high pressure water entering cracks and voids in the surface.
Most readily available consumer units (commonly found online or at hardware stores) are electric- or gas-powered. The electric ones plug into a normal outlet and use/produce cold water, and typically deliver pressure up to about 2000psi. Gas-powered units can often deliver twice that pressure, though due to the hazardous nature of the gasoline exhaust they are unsuitable for enclosed and/or indoor areas. Some models can generate hot water, which can be ideal for loosening and removing oil and grease.