Cel shading is a subset of toon shading. They are very similar techniques for non-photorealistic rendering, and the two terms are sometimes used interchangeably. Both give 3D computer graphics the flatter, hand-drawn look of comic books or cel-animated cartoons.
Toon shading uses flat shade colors on top of a shade color and can create other stylistic looks, such as sketching, ink and oil painting, and a low-poly toon shader. Cel shading does not use shade gradients. Instead, it has only two or three tones to its shading. Its purpose is to specifically simulate the 2D look of anime, comic books, and cartoons for 3D graphics.