W-9 forms and tax resale certificates are informational documents that you give to companies with whom you do business, but they serve different purposes. The W-9 is a federal form that discloses your employer identification number for tax-reporting purposes. Tax resale certificates are issued by states and contain a resale number, but their primary purpose is to establish that your business doesn't need to pay sales tax.
The W-9 is a one-page form created by the Internal Revenue Service. On it, you enter your company's contact information and its Employer Identification number -- or, if it doesn't have one, your Social Security number. W-9s must also be signed and dated to certify their accuracy. Although the Internal Revenue Service doesn't get a copy, the W-9 is still an official tax document.
Generally, when you do business with a business that will need to send you a 1099 informational return at the end of the year, it will give you a W-9 to fill out. It's that business' responsibility to have one on file, so the business should be proactive about complying. However, if you need one for your vendors, you can download it from the IRS website (see link in Resources).
A tax resale certificate is a document that contains your business' contact information and its resale number. Your business uses it when it is purchasing items that it will be reselling. With your resale number, the vendor that sells you those inventory items doesn't have to charge you state sales tax, since you'll be charging it. If you're a reseller, it's your responsibility to provide your certificate to your vendors.
The process of getting a certificate varies from state to state. Generally, you apply for a resale number with the state agency that is in charge of sales taxes. Once the number is granted, some states will issue you a certificate that you can make copies of and share with vendors. Other states just give you the number, leaving you to make up your own certificate that you can share.