As all engines require fuel to make them run, anything that impedes the flow of fuel can cause starting or running issues.
The fuel filter's purpose is to filter out potentially harmful dirt or debris that could enter the fuel system and cause these symptoms.
Modern engines' injection systems have incredibly tight tolerances and any contamination can prove to be very costly to rectify, so the filter is a component that's important to replace regularly.
Older petrol engines, particularly those with carburettors, are less fussy when it comes to fuel, but if you get a clogged, or blocked fuel filter you'll still experience problems, so neglect it at your peril.
There is also the issue with water contamination, particularly with diesel engines. Many diesel vehicles' fuel filters have a drain tap at the bottom of the housing that allows any water to be drained off.
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There are two main types of fuel filter you're likely to come across, but both function in exactly the same way: fuel is passed through a filtration medium (often made from a special paper) which removes any deposits.
Some filters are simple plastic or metal housings around a filtration medium, and these aren't intended to be serviced, instead you simply replace the entire unit.
Others are cartridge-style with a replaceable filter housed within a metal container. The lid of the container can be removed, and the filter swapped for a fresh new one.
It's worth noting that some newer cars don't actually have a filter and simply rely on a strainer that's attached to the pump, or fuel pick up pipe, so if you can't find yours – don't panic!