A 401(k) Plan is a defined contribution plan that is a cash or deferred arrangement. Employees can elect to defer receiving a portion of their salary which is instead contributed on their behalf, before taxes, to the 401(k) plan. Sometimes the employer may match these contributions. There are special rules governing the operation of a 401(k) plan. For example, there is a dollar limit on the amount an employee may elect to defer each year. An employer must advise employees of any limits that may apply. Employees who participate in 401(k) plans assume responsibility for their retirement income by contributing part of their salary and, in many instances, by directing their own investments.
Source: U.S. Department of Labor