A CUSIP number is a security's identifying number, not unlike a product's catalog or bar code number. Its nine digits are assigned by Committee on Uniform Security Identification Procedures (CUSIP). The first six digits identify the issuer, such as a company, while the last three digits reveal exactly what kind of security it is, such as a particular kind of stock or bond. It's good to have CUSIP numbers handy when you're transferring shares of stock or if you've lost your certificates.
Learn more about CUSIP numbers from the horse's mouth, at CUSIP.com, where you'll find everything you ever wanted to know about CUSIP (and probably more).