How are monthly mortgage payments calculated?

I ask because i have just used a mortgage calculator online and they have worked out this: To borrow �100,000.00 over a term of 15 years at a monthly calculated interest rate of 4.89% gives: Repayment Mortgage Monthly Repayment �785.08 and Interest Only Monthly Repayment �407.50 Thanks for any help. http://bit.ly/lRJC6H