I get this question a lot from people. What does InDesign do, why do I need Illustrator, can't I just do this all in Photoshop, why do I need InDesign for this, I like Photoshop for all my design needs. The list goes on, but the websites out there do it with lots of words, so I thought a very simple infographic would work great.
Illustrator: Greatfor Logos and logotype creating and editing, vector graphics and illustrations, infographics and motion graphics. Business cards, letter head, stationery.
Not for image editing or anything pixel based. Large multi-page publications are better suited for InDesign than illustrator. Audio, video and animation are a no-go in Illustrator.
InDesign: Greatfor Books, reports, brochures, flyers and all sorts of print materials. InDesign makes large documents easier to format, interactive presentations and PDFs simple to create and can also be your go-to for ebook and digital publishing.
Not for image editing, anything beyond the most basic drawing of shapes, never create a logo in InDesign, not for UI/UX design and does not have good support for image filter or effects.
Photoshop: Greatfor Image editing, compositing, color adjusting and matching, adding special effects and filters to images, processing camera RAW files, creating and editing Web and motion graphics as well as software, web, mobile and UI design.
Not for logo design, vector illustration, type or vector logos, any type of print materials that contains more than basic text. Never used for brochure or book layouts.