Depending on your loan agreement, excess payments may be credited towards interest or it may be credited towards principal. If the latter, then the loan ends up being paid off sooner and you pay less in interest overall. In some loan agreements it might be possible for the lender to still get the original amount of money in the form of interest by recalculating the interest rate if the principal drops too much.
With my mortgage I made sure that I had the option to pay directly to principal and that the interest rate could not be recalculated (it was a fixed rate loan). Thus I paid off a 30 year mortgage in 16 years.
With my cars I simply waited until I could afford to pay off the balance of the loan and did so in one swell foop. That ended the loan so I paid no further interest. The lender is not fond of that because they count on that interest income.
In other words, you need to go back and read all the fine print on your loan document to see what you agreed the lender can do.