Pressure washers work by increasing the low pressure available from your tap into a high pressure (up to 140bar) jet. This high pressure water stream can be used for a range of cleaning jobs.
When comparing models, the main differences are the motor size together with water flow and pump pressure. A higher wattage motor makes for higher pressure (measured in PSI or BAR) meaning a more powerful washer. Also check the hose length, make sure it's long enough to reach the length of your car so you can clean the whole vehicle without having to move the pressure washer.
There are some great attachments available for pressure washers including snow lances which mix detergent with air to create a thick foam.
Also, did you know you can still use a pressure washer during a hose pipe ban?