The difference has to do with when you pay taxes on your contributions:
A Roth IRA is for after-tax dollars and allows you to make tax-free withdrawals when you retire (after age 59 1/2).
A Traditional IRA is for pre-tax or tax-deducted dollars and allows you to pay taxes later when you withdraw (usually when you retire after age 59 1/2).
A Rollover IRA is a special type of Traditional IRA for rollovers. Rollover funds in this type of IRA can sometimes be rolled back into an employer-sponsored 401(k), if the plan allows for it. You can still contribute to Rollover IRAs the same way you would to a Traditional IRA.