Illustrator = Vector art. Useful for all sorts of things like logo design using spot colors, overprints, vinyl cutting and design, etc. Exportable as AutoCad .dxf and .dwg files. Vector art is infinitely scaleable meaning you will never lose resolution no matter how big you scale it. This is the program for logo design, you can always raster (convert vector to pixels or a bitmap) from Illustrator.
Indesign is a page layout program. It’s main advantage over creating layouts in Photoshop is that you can link to files creating a layout which allows you to modify pages without the massive filesize of loaded images. Plus, you can then export proofs and other layouts, insert pages, etc. Dedicated page layout program.
Photoshop obviously is a bitmap editor. Not good for commercial logo creation or printing as everything will have a set res.
What’s nice is Adobe bought up Macromedia, and incorporated a lot of their programs features and functionality into their products.