To many graphic designers this question may seem super silly. And now that I am used to using these software programs daily, I am surprised I ever asked this question. But I did. I found I would get confused about when I should use the different programs, and what their main function was.
Now, I am still a beginner in many ways. I am sure there are HEAPS of functions that I have never used, and don't even yet know exist, on these awesome products. However over the past year I have used them almost every day, done heaps of reading on them, followed tutorials and completed assignments, and now I do feel I have a good handle on when to use what.
Here is how I use the programs:
Illustrator - best for illustrations and logos. Creates a vector file, which essentially means the file can be scaled up or down within the program without loss of quality (or that annoying pixelation problem).
When do I use illustrator? When I am creating logos and branding. All my brand boards were made in illustrator from start to finish. I also use it for creating all my blog header images- this could be done in photoshop (and I have made them there too) but I just like how Illustrator works, and so tend to use this instead. (This is probably my favourite program- I use it almost all the time and rarely close it)
Photoshop- best for editing photos (der! But still, it can get confusing as some people use photoshop for web design and illustrations!). Can also be used to make images (like my blog header, or even more complex images) When do I use photoshop? To edit all my images taken on my camera, and to resize images to smaller sizes to upload to my site.
Disclaimer: This software is the one I feel I have the least exposure to and least practice with. I know there is SO much more I can do with it then how I use it, but still- this is a guide based on my knowledge and reading only, so being honest- I do use it a lot, but only for pretty simplistic photo editing.
InDesign- best for print layouts. It handles large volume of text really well. You can do some pretty cool things on this program, like thread text between various boxes that you can resize, move and reshape, without losing the text. Use this program for things like magazine layouts, print advertisements, flyers, or books.
When do I use inDesign? Pretty much for creating assignments for my diploma, when I need to make layouts for magazines or advertisements.
Not the most comprehensive guide to these programs, but hopefully that quick overview gave you a bit of a head start and help. I know when I started off I was looking for just a really quick way to know what each program was for, and so I thought I would share what I learned in case I can help someone else :)