I've been a publishing professional since the early 1990s, and as a result, was weaned on QuarkXPress. Sure, Quark had its problems — chief among them being the company's unresponsiveness to its customers — but it was light years ahead of its competition, Aldus' (and then Adobe's) PageMaker.
Anyone reading this article is likely to be as old school, or nearly as old school, as me, so I won't be pussyfooting around technical terms like master pages or kerning. You're either a professional freelance designer looking to make the switch to an all Adobe workflow, or in a corporate design environment where you're being forced to migrate from Quark to InDesign.
Whatever your situation, have no fear. InDesign bears no resemblance to its predecessor PageMaker, which was so awful in production environments it earned the nickname "PageWrecker," and prepress ops would convert PageMaker files to Quark because the finessing needed to make them print was less than if you ran them from their native application.
Adobe listened to users, and from the initial CS launch to CS4, has implemented features designers had longed for for years. InDesign got layers before Quark did (I have to confess, the last version of Quark I've used is 6), tossed in some useful and usable minimal drawing tools, and seamlessly integrated with Illustrator and Photoshop.
The transition from Quark to InDesign will be nearly seamless. It uses text and picture boxes (frames, in Adobe-ese) just as Quark does. InDesign has master pages, style sheets, and linking (threading) text boxes, and a Collect for Output (Package) that puts Quark to shame.