An illustration by definition is a visualization such as a drawing, painting, photograph or other work of art that stresses subject more than form. The aim of an illustration is to decorate textual information (such as a story, website, poem or newspaper article) by providing a visual representation.
Illustrations usually used in web design to attract more attention by using characters, scenes or other abstract forms. In this article I have collected 40+ excellent examples of using illustration is web design.
Links are listed in no specific order. If you are a website designer and designed a website with illustrations please be kind and share it with the rest in comments.