While each lender decides what credit score it considers a good risk, if you're credit score is generally 650 or better, you will probably be considered a good credit risk (meaning, you're likely to pay off a loan on time and in full), and you'll therefore qualify for a prime (i.e., desirable) interest rate. If your credit report rating is between 620 and 650, you may also be qualified for a desirable loan, but you may need to provide the lending institution with additional documentation to prove that you're creditworthy.
Credit scores below 620 put you in a greater risk category in the eyes of lenders, which means you'll probably have to pay a higher interest rate (also known as a "subprime" rate) on your loan. A low credit report rating will also limit the amount of credit or the size of the loan you're able to receive.
Even just a few points one way or the other in your credit score could mean a difference of thousands of dollars when you're paying off a mortgage over 30 years. And the savings that a high credit score typically offers can then be applied toward retirement savings, college funds, cars, vacations, healthcare needs or even day-to-day expenses.
more info on credit & your purchasing power: http://credit.privacymatters.com/credit-articles/credit-rating.aspx
more info on credit: http://credit.privacymatters.com/learning-center.aspx
Good luck, hope this helps!