Both types of plans offer a tax benefit.
A traditional IRA allows you to invest pre-tax money into the account and it grows tax free. Once you withdraw the money it then gets taxed as though it were income based on the amount you withdraw for that calendar year. A Roth IRA has you invest post-tax money and also grows tax free. However, when you make withdraws in retirement that money is then tax free.
Neither plan is right for everybody. If you have a very high income now and plan on being in a smaller tax bracket later when you'll be making withdraws then the traditional IRA is better. If you will be in a higher bracket later, then the Roth IRA will serve you more. Depending on the way you manage your retirement investing you can likely invest in both if you are unsure as to which would be better. The same type of investments should be able to be nested within each type.
