A 401k is a form of retirement savings plan that many firms in the United States provide. It allows employees to save for retirement by contributing a portion of their pre-tax income to the plan, which is then invested in a variety of investment options such as mutual funds, stocks, and bonds. The contributions and any earnings on those contributions grow tax-free until they are withdrawn in retirement. Some employers may also offer matching contributions, where they contribute a certain amount of money to the plan on behalf of the employee.