Traditional IRA: contributions are pre-tax, tax paid upon withdrawal. Theory is you will contribute more to avoid taxes while earning, compound growth works to your benefit, but now pay taxes on the full amount in the IRA as you withdraw it.
Roth IRA: contributions are post-tax, no tax paid on withdrawal. Conventional theory for typical earnings level is that you will be in a higher tax bracket when you start withdrawing then when contributing, so pay the taxes up front, let compound growth do its thing, then take out the entire amount with no taxes.
Ergo, if you can get contributions into a Roth that you know will show enormous amounts of gain, you're going to be able to pull that out entirely tax free. That would not be true for a traditional IRA or 401k, where you will pay taxes as you withdraw.
