Most traditional retirement plans such as: 401(k)s, 403(b)s, 457s, IRAs, SEPs, ) are tax-deferred. This means that taxes are due once the money is accessed in retirement.
Keep in mind that we are currently in one of the lowest tax-brackets in history with Social Security and Medicare being underfunded, and our government amassing a larger and larger deficit (visit www.usdebtclock.org). Mathematically, taxes in the future will have to go up to fix these challenges. Additionally, most people during retirement will have much less tax write-offs (mortgage, children, 401k contributions).
It is important to have a retirement plan that not only provides income, but also lets you keep as much of it as possible. Diversification between tax-deferred and tax-free income should be an important part of retirement planning.