They are both very similar and share a lot of features
Illustrator is for one off documents and illustration
Indesign is more for magazines and multi-page docs
Saying that, what you learn in one app can easily be b applied to the other
I am an old veteran of illustrator, and found that my illus skills meant that I already knew about half of Indesign
so, in short, use illus for drawing and InDesign for documents, but the skills you learn in one will often transfer to the other