There are plenty of differences, but I'll let those who know better than me give you the differences. Indy cars came about to try and hold down the cost of racing in the open wheel ranks. Those who were opposed formed a new series and continued in the direction they were heading while the IRL (Indy Cars) began using I believe crate motors and raced on ovals while CART, the new series raced on road courses. Just like Formula 1 cars are different than Champ cars, so are the Champ cars different than the Indy Cars. It's all in the technology of the cars and the cost of the exotic materials they use.