Iridium is an element, number 77 on the table of periodic elements. Platinum is number 78, and they are both metals. Conventional spark plugs are made of copper which is number 27.
NGK also has iridium plugs out now, they are more then twice as expensive as Bosch and NGK platinum plugs. They're probably not worth it now, but will come down in price as they become more popular. Platinum plugs caught on quite fast so iridium plugs will probably too. What's after iridium plugs... Osmium?