Photoshop is for image manipulation, so you have an image already and you want to change it.
Illustrator is for creating new art, you start with a blank canvas and make something. (While you can do this in Photoshop as well because it is so powerful, Illustrator is actually better at this. But by "better" I mean if you've never used Photoshop nor Illustrator, you'd want to go to Illustrator for creating art from scratch. But if you are much better at Photoshop than Illustrator, then you may just end up sticking to Photoshop)
InDesign is a layout application. This is for publishing magazines, newsletters, and the like. (Again, you could use Photoshop to do this, but InDesign is much better at it).
The fact is, you can do just about anything in Photoshop, but other applications will do some of the tasks faster and with better end results, but only if you learn how to use them. We have plenty of free videos on Adobe TV to get you started.
Illustrator allows you to have multiple (if you want), live, editable stokes on paths. I also like how you can have nested hierarchies of objects and parts of objects within individual layers.
To dovetail on Brett's comments, I will add my one personal preferences for programs:
Photoshop - pixel-base, often photographic, Image creation, or manipulation.
AI - Vector-based illustration, that might, or might not be Rasterized in the end. Also very useful with some Type handling.
InD - When I am doing page layouts, usually for printing. Combines excellent Type handling, Vector Art and pixel-based Images well. This program allows me to put ALL the parts together for the best final output.
Thanks guys.. that helps.
So, as a photographer, I can of course see using PhotoShop and then InDesign for things like brochures, books, catalogues, posters, etc. Still not sure about AI though.
Bill.. when you say 'Rasterized' - what exactly does that mean?
Vector Art is defined mathmatically, where Photoshop is basically pixel-based (there are execptions, such as Type, many Smart Objects and Shapes, but in the end those will most likely be Rasterized, i.e. changed to pixel-based).
If you are designing, say a logo, AI would be the prime choice. That logo design, using Vector, could be resized ad infinitum, with no loss in quality - until one Rasterizes that AI file, in say Photoshop.
Personally, I use two Adobe programs most often: Photoshop and Premiere Pro. I am first a photographer (hence Ps), but also do video production (hence PrPro).
However, I do enough logo, and other Vector work, that I could not live without AI, though it does fall down MY list - behind After Effects, which is a great animation and compositing program for video, and then Adobe Encore, for authoring DVD's and BD's.
Still, I do enough printing of brochures, business cards, envelopes, etc., that InDesign gets used too.
While there IS cross-over, I still try to use the best tool, for my needs. One can sort of force an adjustable wrench/spanner for bolts, but usually a socket set is better. One can drive a nail with a screwdriver, but a hammer is usually better. It depends on the task, and with most of my Adobe programs (all in a suite), I can seamlessly work between them, utilizing each program's strength.
If that does not answer your question, please do not hesitate to ask for clarification.
Certainly inDesign is a whole different Animal to the image apps like Photoshop and Illustrator. As Bill says, inD is good for print, as text and shapes will be scalable an sharp at any print size, but that applies to illustrator as well. But inD is great for outputing PDF documents like magazines, and has lots of tools to make life easier.
Rather Shamefully, I paid NZ$1000 for inDesign CS5 a while back, but all this time later, I am still using Pagemaker for the Camera Club newsletter that I edit. inDesign has some major advantages over Pagemaker, like video in a PDF, but my excuse is that I am always too busy to learn a new app. Quite how I am going to cope with a full CC subscription, I am not entirely sure, but I'll be giving it my best shot.
That was perfect Bill. Thanks. Kinda what I suspected, but I've never seen it put to words. I've been a professional still photographer for 25+ years, but toying with video now so Premiere Pro will be the next app for me to tackle.
In another lifetime, long before video made inroads, I was an engineer, and left the profession to return to school. I obtained another degree in Cinematography, but put myself back to school by doing still advertising photography work. When I walked out of university, I had a great number of still photography clients, and a good portfolio. I only did a few cine projects.
Fast forward about 25 years, and I retired from advertising photography, and rediscovered my cine-side, but it was with digital videography by then. I stumbled around a few NLE (Non Linear Editor) programs, and hit on Premiere. I never looked back, though still did a bunch of work in my beloved Photoshop. Now, I have retired from video, as well, and only work on personal projects, and test for the Adobe Forums.
Over the decades, I have used most of Adobe's programs, and still use several. Along the way, I did quite a bit of design, so did logo design, and also project printing. That is where I came to PageMaker, and then InDesign.
Good luck, and let the fun begin!