A 401(k) is a type of retirement savings account in the U.S., which takes its name from subsection 401(k) of the Internal Revenue Code (Title 26 of the United States Code). 401(k) are "defined contribution plans" with annual contributions limited (currently to $17,000). Contributions are "tax-deferred"—deducted from paychecks before taxes and then taxed when a withdrawal is made from the 401(k) account. Depending on the employer's program a portion of the employee's contribution may be matched by the employer.
Read more about 401(k): History, Tax Consequences, Withdrawal of Funds, Plans For Certain Small Businesses or Sole Proprietorships, Other Countries, Risk