A 401(k) is a retirement account offered by your employer. You save for retirement straight from your paycheck, and your money grows tax-deferred or tax-free. Many employers also add matching contributions.
Traditional 401(k): You contribute money from your paycheck before taxes, which lowers your taxable income today. Your balance grows tax-deferred, and you pay taxes when you withdraw in retirement.
Roth 401(k): You contribute money from your paycheck after taxes. Your balance grows tax-free, and you pay no taxes when you withdraw, as long as you follow the account rules.