A credit score is a number between 850-300 that ranks your creditworthiness. Your credit score is calculated by the three separate credit bureaus Experian, Equifax and Transunion. Each credit bureaus can have a different score. It is important know all three scores to help identify errors, and give you an overall average.
With that being said, AnnualCreditReport.com is the ONLY authorized source to get your free annual credit report under federal law.
*Beware of sites that claim to have free credit reports and free scores, they will usually lure you into subscribing to some kind of credit monitoring program that will end up costing you money.*
A good website to purchase your credit score is MyFico.com. They offer a fairly encompassing evaluation of your credit, including your all three credit scores for around $50.00. MyFico.com is also highly recommended by Suze Orman.
It is good to check your credit at least once a year.