What is an IRS W-9 form, and why is it necessary to submit to a bank in order to release my funds?

An account was opened in my name many years ago by my father, with himself listed as the custodian and me listed as the minor (but primary account holder).

Now I am of age and trying to retain these funds, but the agency holding the money will not release without a W-9 form.

I would simply fill out this form and return it to them, however I sent them the paperwork they originally requested (name, address, SSN, etc. with bank guarantee stamp) only to be told they now need a W-9.

Before I send the W-9, I would like to know a little more about what they can do with it, and why it would be necessary. I have gotten the basics from Wikipedia here http://en.wikipedia.org/wiki/W-9#W-9 but a little more detail would be great.