Keep your W-9 handy...

As a freelance photographer, you will be asked for a W-9 to be filled out and faxed or emailed over.  I get that ALL the time.  Not only from new clients, but also from regular clients who are updating their records.  Just store this PDF right on your computer's desktop and you'll be ready to print it, fill it in, and fax/email it at a moment's notice:  

http://www.irs.gov/pub/irs-pdf/fw9.pdf?portlet=3

Why do you need to submit a w-9 in the first place?  In real simple terms, if a client hires you to do work, the government wants to keep track of that money.  They want to tax you on it, so they get their fair share.  The w-9 allows your client to send you a piece of paper that comes in real handy around tax time, called a 1099.  Nothing too fancy, just a statement of what you were paid throughout the year.  This gives you a better explanation:  

http://www.wisegeek.com/what-is-a-form-w-9.htm