Illustrating is like giving a drawing a purpose.
You can illustrate a letter, a story, or a whole book.
You can illustrate a powerpoint presentation for work, or a handbook with instructions. You can illustrate a boring study book in such a way, that students will find it easier and nicer to study and take more in.
You can illustrate a children´s book so that a child will remember it fifty years down the track. And of course, you can illustrate a card and send it away.
You can also illustrate a point you want to make or a joke.