A fender is the part of a car that protects and frames its wheels and keeps mud from splashing it. Most modern vehicles have fenders on their front wheels.
When you get into a minor car accident, you can call it a "fender bender," and it's possible that your fenders did get slightly dented. Another kind of fender guards or protects a ship, train, or streetcar; in the case of a train, this is also called a "cowcatcher," and it can actually push debris off the track to keep the train from hitting it. This 13th century word is a shortened form of defender.