Are you talking about 401K?  If it is, the advantage to put there is it is pre-tax.  But you will have to pay income tax when you withdraw it.  The idea is when you retired, you should be in a less tax brack then when you are current in.  YOu can't use it until your retirement age.  Early withdraw will have penalty.  your company should provide you the options of what kind of mutual fund or any other investment you are available to you.  The investment gain is just like any other investment, depend on the funds performence. If you are going to just save to bank for retirement, then you should use 401K as much as allows and affordable.  If you are going to invest your $ elsewhere, you can evaluate which is better for you.

BTW, your companys HR and the 401K broker should provid you details information.  And the broker should help you alocate your fund to best suit yiour financial style.