On the copper spark plugs the electrode from where the spark comes from is copper, normally pretty cheap in price. ($1.49)
Platnium plugs have a platnium electrode that lasts longer than most spark plugs, good for those people who don't really look after there car that well, they last typically 100,000 miles. The point is that manufactures put these in new cars to cut maintainance and service costs to those vehicles with hard to reach spark plug wells. They typically cost about $5.00 a plug I believe.
My advice, save your money, buy the "normal" copper plugs and replace them more often. If you would like more info NGK has a website with some good information as well as illustrations.
www.ngksparkplugs.com Oh and I paid $20 for my platnium plugs and $25 for my 7mm plug wires.
tyler
[This message has been edited by Tyler Hodgson (edited September 05, 2000).]