Contributions are made on a pre-tax basis, meaning you can deduct that amount from your taxable income if eligible. In addition, the money in the Traditional IRA grows tax deferred, meaning any income/profit from the Traditional IRA investments are not taxed. Instead, taxes are deferred on these funds until withdrawn.
Anyone under age 70 ½ with U.S. earned income is eligible to open a Traditional IRA, but there are some restrictions as to who can deduct the contributions. There are income limits that are used to determine how much of the contributions are deductible, if you or your spouse are participants in an employer plan.
If you are over age 70 ½, you can still open a Traditional IRA, however, you cannot make any contributions. In order to fund the account, it would have to be via transfer (from an employer sponsored plan) or via rollover (from another existing IRA).