1. The amount of money you borrow
2. The length of time you borrow the money
3. The interest rate applied to the loan

Amount:  This is simply the "bottom-line."  It is the amount that is due to the seller after all fees have been added (tax, title, license, warranties, etc.) to the selling the price of the vehicle, minus the amount of your down payment (trade equity or cash). 

Length of Time: Financing for long periods of time will lower your payments, but the total amount you pay for the vehicle will be higher (perhaps much higher).  It is always best to finance for the shortest period of time you can afford.  The average length of time between trades is typically thirty-months, so loans longer than thirty-six months are not advised unless you plan to keep the vehicle for a longer period of time.

Interest Rate: The interest rate on any loan is always based on risk.  The higher the risk, the higher the interest rate.  Risk is determined by several factors including, but not limited to, the credit-worthiness of the borrower.  If the borrower has bad credit, it makes it more difficult to get financing...especially at low interest rates.  There are however, factors that influence risk that the borrower, even a bad credit borrower, can use to reduce risk and get a lower rate. 

• Equity:  Since an auto loan is a secured form of lending, the lender will look at the amount of equity in the loan.  Equity is the difference between what the vehicle is worth on the open market and the amount of money lent.  The more equity in the loan the less risk to the lender.  Hence: Having a large down-payment will reduce the risk...and the interest rate.
• Term:  Short-term loans are less riskier than long-term loans, so always finance for the shortest time you can an afford.
• Warranties & Insurance:  Warranties and insurance reduce risk by protecting both you and the lender against unforeseen events.  It is against the law to require such products as a condition of a loan, but you should know that these protections reduce risk for both you and the lender...and makes qualifying for a loan (and a lower interest rate) easier.

Terms and conditions of any loan are subjective at best; and like all economic principles, are based on supply and demand.  But understanding how financing works should help you maximize your ability to get financing and the lowest rate possible.