SparkPlugs
Normal Spark Plugs
Spark plugs will periodically need to be gapped, which ensures that they are creating the hottest spark possible and therefore burning all the fuel in the cylinder. Spark plugs are also very good indicators for a number of problems that can occur with combustion. If read properly spark plugs can show if you are burning oil, running too rich or too lean of a fuel mixture, and even if your engine is igniting the fuel before the spark plug discharges. This page has a number of pictures which can help you diagnose engine problems by the appearance of the plugs.
Iridium Spark Plugs
Iridium plugs, made by NGK, offer benefits over ordinary spark plugs but also require special care. Iridium plugs create a hotter spark and last longer than normal spark plugs, so once installed you probably won't ever have to replace them. They should not require maintenance but will suffer from the same problems as normal plugs if the engine's combustion isn't tuned. Some users recommend revving the engine above 8000 periodically to burn any uncombusted fuel off of the plugs. This is recommended particularly if your bike frequently runs with the choke on.
External Links
Dan's Online Motorcycle Repair Course has a very helpful page on spark plugs
NGK Iridium Plugs for the GS500
Wondering how your engine is doing? Look at your plugs