Pre-Tax Contribution

A retirement contribution made before income tax is applied, with tax paid instead when money is withdrawn.

Example

1,000 of salary goes into the account in full, and withdrawals are taxed later.

It beats a post-tax contribution only if your tax rate on withdrawal is lower than your rate when contributing.

Post-Tax Contribution — A retirement contribution made from income that has already been taxed, with qualifying withdrawals typically tax-free.
Tax deferral — Postponing a tax liability rather than removing it, so the money that would have gone in tax keeps compounding in the meantime.
Marginal Tax Rate — The rate of tax applied to your next unit of income.

The account is a wrapper; what you hold inside it is a separate decision — A retirement account is a legal container with tax rules and access limits. It is not an investment itself, and the choice of wrapper and the choice of holdings are two different decisions. (9 min)
Pre-tax or post-tax is really a bet on your tax rate now versus later — The two main account structures give the same result if your tax rate is the same when you contribute and when you withdraw. The choice turns on whether you expect your rate to be higher or lower in retirement. (12 min)