403b is the 401k for nonprofit corporations such as most hospitals. Look into your particular plan to find out if your hospital matches any part of your contribution. The hospital I work for now contributes .5% up to 10% of my contributions. So I have an instant return on my money of 5%. The money comes out pretax and the money is tax free until I begin to withdraw it, so my tax savings are around another 5% I think.
You also need to check into when the money is vested. Mine is vested from day 1. Some plans have a waiting period for vestiture of up to 10 years. The money you put in is always yours, but any matching from your employer may be removed if you leave cmployment with that entity.
Where you put your money may also be up to you in some plans. AS 3rd shift guy says, some plans allow you to pick funds of different types but usually from the same company. Don't invest according to past performance of the fund. Determine the level of risk vs. return you are comfortable with and invest accordingly. There are a lot of online resources that can help you decide, but I recommend The Motley Fool at www.fool.com.
It's free and very very forthright. Good luck, Gary