What is Adobe InDesign? The Descriptive Answer
Graphic design can be roughly split into web design and print design. Adobe InDesign is a program used to lay out documents for print. It's part of the holy print design triumvirate of Adobe: Photoshop, Illustrator, and InDesign.
Each of these programs has a role to play in producing pieces for print. Imagine you are assigned to create a brochure incorporating photos, line art, and copy (blocks of text). Photoshop is the program you use to edit photography, or raster art (images composed of pixels). You'd prepare the photos for the brochure in Photoshop, adjusting color, cropping them, sharpening them, and the like. Illustrator is the program you use to edit the line, or vector, art (images composed of points and Bézier curves).
The logo of the company you are producing the brochure for is likely to be vector art, and if you had to edit the logo, you'd do it in Illustrator. Of course, this could easily lead to a discussion of the sanctity of logos and how you should never have to edit a company's logo, but let's just say that some companies aren't quite on top of things and will provide you with a crappy version of their logo pulled from their website, in which case you'd offer to re-create it in Illustrator for them so their brochure doesn't look like garbage. Just saying.
Okay, so now you've got all the photos ready, you've got the logo they "provided" you by pulling some 75 x 50 pixel 72 dpi monstrosity off a website cleaned up and redrawn in Illustrator, you've been given the copy, and you've sketched out a layout. Where does it all come together?
It all comes together in InDesign, that's where.