A 401(k) is an employer-sponsored retirement savings account. When you open an account, you can make pre-tax contributions from your paycheck into the account. Your investment choices depend on the options your employer selected, but you’ll likely be able to invest in mutual funds or exchange-traded funds. You don’t have to pay income taxes on your 401(k) contributions or earnings until you start taking withdrawals. In most cases, that won’t be until you retire.