You always pay regular income tax (unless this is a Roth option account in your 401k plan).
If you were under age 55 when you left the company, and you are under 59-1/2 now, you will also pay a 10% penalty. If this is a Roth option account, you will pay income tax and a penalty on any earnings you withdraw, but I don't believe the principle contributions are taxed.
401k plans do not have the same penalty exceptions that IRAs have. There are certain reasons you can withdraw money and be exempt from the penalty, and some of the reasons apply to IRA but not 401k plans. In that case, you can do a rollover from the 401k to an IRA and then withdraw the money from the IRA.
