The canter, or lope, is a three beat pace. It is characterized by a movement when all four legs are off the ground. This is called suspension.
The sequence of movement for a canter to the right is:
1. left hind
2. right hind and left fore together
3. right fore
If you are cantering to the right, the right fore leg "leads" - it is the leg that extends the most in front. For the canter to the left, the left fore leg leads. Cantering on the wrong leg or lead is when the horse strikes off incorrectly with the wrong leading leg. Sometimes this is done intentionally at higher levels of riding as a suppling exercise, and is called a counter canter.
Watch the animated horse cantering below. If you look closely, you will notice that his right fore leg is leading his left fore leg, and that the right hind leg is leading his left hind leg. When a horse is very young or a rider is unbalanced, a horse may "cross canter." A cross cantering horse has the right fore leading his left fore, and his left hind leading his right hind. This also may occur when asking for a flying lead change on an unexperienced horse, or an unbalanced rider. Cross cantering is strongly discouraged. It is uncomfortable for the horse and means he is unbalanced. It's not so comfy for the rider either!
Most people when they are learning to canter have to drop their eye to the horse's shoulder to check whether they are on the correct lead. However, dropping your eyes often causes you to drop your head, which in turn throws the horse off balance - which then makes it difficult for the horse to pick up the correct lead. With experience a rider can feel when the lead is wrong. A good exercise one can do to practice "feeling" the lead is to have a knowledgeable person who can observe your canter. Pick up the canter, and say what lead you are on, (without peeking!) Then have your helper tell you whether you were incorrect or correct! Also make sure you know how to correctly ask for the appropriate lead! One way is to turn the horse's nose slightly toward the center of the ring, keeping your inside leg (right leg if you cantering to the right, or clockwise) on the girth, and then squeezing behind the girth with your outside leg.