technically you never need to replace your spark plugs, but ideally you should replace them once 50000-70000KM
It's a spark plug - so it makes sparks. During an engine cycle when the cylinder is injected with air-gasoline mixture and is at the top location (maximum compression), the spark plug creates a spark that ignites the fuel mixture, which causes the piston to shoot downwards, which keeps the engine spinning.
... I'd say they're mostly the same. But iridium/platinum is said to have longer life and creates better sparks - saves fuel and more output power.
they're kinda like a screw that screws into your engine... but you NEED a torque wrench otherwise you'll end up replacing your headers.. they have to be torqued to a specific torque. and don't forget those anti-seize stuff...
typically 2 spark plugs range from 10-30 bucks