Basically a pressure washer boosts the pressure coming from you water supply.
You attach a garden hose to the tap and then attach the other end to the inflow of the pressure washer.
The urban supply pressure in the UK, is approximately 4-5 bar of pressure. This is what will be coming from your tap.
When you turn on the supply and switch on the washer, an electric or petrol motor powers a pump that takes the water and drives it through the machine at speed, producing pressure up to 150 bar, depending on your machine.
This powerful jet of water is then directed through another hose and out of a high pressure nozzle as a powerful jet.
This jet can be directed through a number of different attachments with nozzles designed for specific tasks.
This method uses less water. A hose at 4 bar for an hour will use around 3,500 litres of water. A pressure washer at only 100 bar will use about 400, potentially saving time and money.