lenght of time they last is different ger

The main difference is the material that the 'tip' of the spark plug is made of - normal ones are usually copper, whereas the other two have tips made out of platinum or iridium.Platinum and iridium tips tend to last longer - copper tends to erode over time so the spark plugs wear out - plus they're slightly better conductors and can produce a spark under more adverse conditions (like under high cylinder pressures on turbocharged cars that run high boost).On an unmodified car that specifies normal plugs, platinum or iridium plugs won't make much of a difference apart from cost; I'd stick with the manufacturer's recommendation and change the plugs as per schedule. On an engine that has been modified with higher compression or more boost, it might be worth springing for platinum or iridium plugs.Of course, on a car where those plugs are specified (usually due to the extended service interval for the plugs) you'll have to use them anyway.