Measurement Units Supported by InDesign
InDesign offers several measurement schemes for your use. The web designers among you might notice that "pixels" isn't offered. This is because InDesign is a tool for print, not online.
Inches, picas, and millimeters are the most commonly used systems. Inches is the most common system used for laying out documents in the United States, with picas a shrinking second. Picas are old school, historically used by printers. A rough equivalency is 6 picas to an inch. Additionally, there are 12 points to a pica, so 72 points to an inch. And if we want to get really deep here, a font's point size corresponds roughly to its physical height. 72-point type will be about an inch tall.
But I digress. Millimeters is used in Europe and many other areas of the world. Paper sizes such as A4 are based on the metric system, so when creating documents for papers such as A4, it's simpler to just use millimeters — the measuring unit that works best.
Changing Measurement Units in InDesign
You set the measurement system you use in InDesign in the Preferences dialog (keyboard shortcut: command-K). Here's an interesting tip, and it applies to all InDesign preferences:
To change units of measurement globally (for this and all new documents), set the new preference without any documents open.
To change units of measurement for the current document only, set the new preference with the document open.
Simply open up the Preferences dialog, click "Units & Increments," and set the measurement system in both the "Horizontal" and "Vertical" dropdown menus. InDesign allows you to use different measurements for horizontal and vertical axes, so it's necessary to set both.
Notice the "Origin" dropdown menu. InDesign can measure across a spread (default), per page, or from the spine. If you've recently migrated from Quark you'll be more comfortable setting Origin to measure each page separately, as that's what Quark did by default.