Cel shading:

A group of shading techniques that represent light and shade in a drawing or painting through blocked-in forms. Characterized by consistently sharp-edged forms of shade and light. Popular among comics, manga, and animation.

Soft shading:

A group of shading techniques that represent light and shade in a drawing or painting through smooth gradation based on the laws of light logic. Characterized by soft transitions between shade and light throughout the image.

Hatching:

Artistic technique of using closely-spaced lines to simulate shading. Hatching is usually done with parallel lines, but can also involve curved lines to illustrate contours, and parallel and intersecting lines (crosshatching) to communicate greater depth in texture and tone. A staple of older monochrome media such as engravings and woodcuts, today it is commonly associated with black-and-white cartoons.

Flat colors:

A basic, solid way to color pictures. They don't contain any hue, shading, or highlighting. Often confused with cel_shading

Monochrome:

This tag is used for images or animations whose entire composition is only or almost only composed of just lines and shades of a single hue. The term is sometimes mixed up with restricted palette when the hues used are similar.