Each EMI is split between interest on the outstanding balance and repayment of the principal. In the early months most of the EMI goes toward interest; over time the balance shifts and more of each payment reduces the principal.
The size of an EMI depends on three things: the loan amount, the interest rate and the tenure. A longer tenure lowers the monthly EMI but increases the total interest you pay over the life of the loan.
For example, a ₹20 lakh home loan at 9% over 20 years works out to an EMI of about ₹17,995. Stretch it to 30 years and the EMI drops, but you hand the bank far more interest overall.