In addition to the above:
You have the option of rolling over the 401k into a private IRA, once you leave service. You can open an IRA at almost any bank or stockbroker. The main difference is that with an IRA, you have complete freedom to invest in anything you want, while the employer may limit the allowable investments in a 401k. On the other hand, large employers can negotiate lower fees so the mutual funds they offer may have lower built in expenses. Also, the rules regarding the 10% penalty for withdrawing funds before age 59-1/2 are somewhat different between a 401k and an IRA.
Some employers may force you to close your 401k after leaving service, especially if the balance is small. In that case, if you roll over the funds to an IRA, you continue to save the money for retirement and don't have to pay taxes or penalties.
Lastly, if you decide to keep the 401k as-is, be sure to log into your account at least once a year to let them know you're alive and keep your phone, address and email updated. If they don't hear from you after a set period of time, your funds could go to your state's unclaimed funds program.
