What s the difference between a cold sore and canker sore?

A few things: Canker sores are red, covered in a white plaque, and usually found inside the mouth on the tongue or wall of the cheek, while cold sores are red bumps that turn into blisters and are usually found outside the mouth on and around the lips. Cold sores are exclusively caused by a herpes simplex virus while the cause of canker sores is often unknown (could be bacteria, virus, allergy, etc.).

Very different: Cold sores are typically lesions on the lip that stem from infections with herpes viruses (nongenital or genital strains). Canker sores are formally known as aphthous ulcers and occur within the mouth. It's not totally clear why, but probably immune and environmental factors are at play.