Canker sores are what I call mouth ulcers. They have a hole in the center, which is the most sensitive and painful part.
I generally don't get them on my gums, though. I get them on my inner cheek flaps, usually close to the gums but not on them.
They rub up against my teeth and gums and cause constant pain. I've heard that biting your cheek can actually cause a canker sore to form, and I believe it, because it seems that not long after I bite myself, I get one.
I rinse my mouth out with warm salt water, and that seems to make them fade a little sooner than they otherwise would. I also make a baking soda paste to rub on them, and though it stings at first, it starts to provide relief after a minute or so.