Updated on April 30, 2008.

IF YOU'RE VIGILANT about your money, your credit score is probably as familiar to you as your home phone number. It affects everything from the interest rates you pay on loans and credit cards to the size of your insurance premiums.

And because one company's product Fair Isaac's FICO score, to be exact has come to dominate the market place for credit scoring, for many consumers a FICO score has become equivalent to "credit score," much like facial tissues are called "Kleenex."

What you may not know is that your FICO score the one you're buying from MyFico.com or Equifax could be quite different from the scores that businesses actually use to make their credit decisions. (Meanwhile, TransUnion and Experian are selling their own scores that have nothing to do with FICO. For more on that, see our sidebar.)

The FICO score was introduced by Fair Isaac in 1989 as a general-purpose score, says Craig Watts, a spokesman for Fair Isaac. It's the figure that mortgage lenders use to help determine your mortgage rates. In 2001, it was made available to consumers as a way to give them an idea of their credit standing.

But usage of this FICO stops here. For other industries, from auto lenders to insurance providers, Fair Isaac has developed more refined scoring models resulting in scores not made available to the public. Auto lenders use a version of FICO called an auto-industry score, while credit-card issuers use a bank-card FICO score to decide whether to approve your credit application. Insurers use Fair Isaac's credit-based insurance scores or the auto and homeowners scores sold by data broker ChoicePoint.

How different are these scores from your FICO score? All credit scores are based on the information in your credit report, so they should be pretty similar, Watts says. But then again, each score looks at that information through its own formula. So some difference is guaranteed.

Potentially, that difference could be large enough to put you in a lower credit tier one with higher interest rates or disqualify you from a loan altogether. "That can be absolutely frustrating for consumers," says Gerri Detweiler, author of "The Ultimate Credit Handbook." "They may think 'I'm a 760 and I'll have no problem getting a loan,' but it turns out the lenders use a different model and their score is much lower."

Does that mean you shouldn't keep a close eye on your FICO score? Absolutely not, Detweiler says. Chances are, it's still a fair ballpark of your credit standing. But it also helps to know what your other scores are and what you can do to improve them. Here are the details.

1. Auto-loan scores
For more details, read our credit score primer

The good news for first-time car buyers or anyone who doesn't have any auto-related credit history: You won't be penalized for not having owned a car before. In other words, the two scores should be similar, at least until you get your first auto loan.

Auto-loan scores aren't available for consumers, so if you're in the market for a new car, your best bet is to review your credit reports for errors, especially if related to the auto industry.

2. Insurance scores
Insurance scores are used by auto insurance and homeowners' insurance underwriters to determine the insurer's risk of losing money. In other words, the risk that the claims you file in a given year will exceed the value of your insurance premiums.

You'd think that your insurance score would be based largely on your past insurance claims. Not so. Whether it comes from Fair Isaac or ChoicePoint, the two companies that dominate the marketplace, your insurance score is based on your credit report. It just weighs the different components that make up your score in a different way. With Fair Isaac's scores, for example, more preference is given to your payment history it makes up 40% of the insurance score, compared to 35% of the traditional FICO score. ChoicePoint doesn't release any details on its scoring formulas.

Where insurance scores can get confusing is the range they use. Fair Isaac's FICO scores run between 300 and 850, but its insurance scores range from 300 to 900. Typically, lenders consider people with FICO scores of at least 700 safer bets and those with scores below 600 as risky, according to Fair Isaac. (The company doesn't release a breakdown of what is considered a "good" insurance score the way it does for its traditional FICO score.) ChoicePoint scores run from 200 to 997; anything above 776 is in the top tier. You can see ChoicePoint's full tier breakdown and purchase your auto and homeowners' insurance scores for $12.95 apiece here.

For advice on scoring some insurance savings, click here

3. VantageScore
For the breakdown, see Experian's VantageScore web site.

$5.95 from its web site, claiming it has more than 700 creditors mostly midsize companies who are testing or already using the score. TransUnion also sells it for $7.95.

Should you purchase your VantageScore? John Ulzheimer, president of Credit.com Educational Services, a consumer education web site, warns that, at least for the time being, very few lenders are using it. "If you ask lenders what score they use, 99 out of 100 will say they use...FICO," he notes. While the credit bureaus may say VantageScore predicts risk better, especially for consumers with little or no credit history, Ulzheimer points out that many lenders if they switch to the new score will take their time to accept it.

"A score is like a good wine," confirms Dennis Moroney, senior research analyst with TowerGroup. "When you produce one it takes time for it to age. Same with the score. It takes time to be tested and implemented." He notes that it will take lenders at least 18 to 24 months to become confident in the score's effectiveness in predicting risk accurately especially in today's tight credit environment.