What does a canker sore look like?

Ulcer: I think "ulcer" is a good word to describe a typical canker sore. In most cases, cancer sores are small--about a size of pencil eraser or smaller. They are not raised or swollen but rather flat or sunken slightly. The color is pale yellow to gray-ish in the center, and the edge may be red and inflamed. On-line "IMAGE" search will be helpful to see photos.

Aphthous ulcers: They are usually round ulcers with a grayish-white colored surface surrounded by a red inflamed circle.