Canker sores are mouth ulcers; for everyone else in the world.

They're usually caused by sharp/abrasive foods and/or bad diet, but can also be a physiological response to stress if you're some who's prone to them.

Cold sores or oral herpes (HSV-1) occurs in 2/3 people worldwide 1 and is about that high in the USA too 2 - though the majority don't display any symptoms. If you have a cold sore and someone takes the piss, chances are they also have HSV-1 and don't know.

Additionally, cold sores can sometimes be confused with in-growing hairs. If you pluck hairs out from around the mouth, sometimes they'll grow back without poking through the hole and can get infected.