What is the difference between an IRA and a Certificate of Deposit? Pros and Cons?

I have invested in Certificates for years and want to learn more about the difference between an IRA and a CD. Can someone explain these in more detail for me? What I am looking for is the best interest rate for my dollar. I don't need the money anytime soon, but don't want to be majorly penalized if I were in dire straights either.