Illustrations are graphic renderings to support text. They support the text. They break up the monotony of text. Many illustrations are used in the printed media like books, magazines, brochures, memos, bulletins, advertisements, wrapping papers and so on. 

In many instances, illustrations work better than photographs because the artist has more control over what to include and what to exclude. 

The humorous illustrator is sometimes called a cartoonist. However, the humorous illustrator is often not called upon to make people laugh. Very often, the subject may just be drawn in a funny way. 

Spot drawings, usually 2" by 2" are drawn to act as a starting lettering for a page or just to mark different pages of a book. They may not be funny at all.