I really like this artcle by Jillian Tamaki: On Drawing and Illustration and the Difference Between the Two
Here are my favorite parts:
I actually think that most people don’t realize or think about the difference between Drawing and Illustration. They think they’re the same. They’re not. Drawing is an act, whereas Illustration (as I define it) is a profession. Illustration can involve drawing (it can expand beyond drawing too, obviously), but it’s actually the act of thinking and problem solving.
Illustration is about fitting your conceptual and aesthetic style to a problem.
The joy of illustration is striking the delicate balance of solving the problem with artistry.