I call myself an expert on Mac OS X but today I stumbled across this rather old feature for the first time.
You might be aware of the Get Info window that appears whenever you select a file in Finder or on the desktop and hit Cmd+I. It shows details about the file, and a preview of its contents. Try it now if this is new to you.
It turns out the Get Info has a more muscular brother called, simply, the Inspector. He appears whenever you hit Option (Alt)+Command+I.
The Inspector has two main advantages. Firstly, if you click on other files, the Inspector will show details for these other files. A Get Info window only applies to the file for which you opened it, and a new one must be opened for each file.
Secondly, the More Info section will show lots of technical details about the file. For an image you’ll be told its resolution, color space, and more. For a PDF you’ll be told how many pages the file has, what software created it, and more. Additionally, in an Inspector window the preview of the file is often (although not always) bigger and therefore more useful.
Update: If the Inspector is used on a folder, you’ll be told in the More Info section the last time the folder was opened.
Take a look at the screenshot below that shows first the Get Info window for a file, then beneath that the Inspector window for the same file. The Inspector is a pretty cool tool and I’ll be using it a lot now I know of its existence. Of course, it’s entirely possible to use the Keyboard component of System Preferences to redefine the Cmd+I keystroke to always call the Inspector.