OK, here’s a little more info. When you press Command-Spacebar, you get a little search box at the top right of your screen. Looks like this:
Type something in, and as you type, Spotlight searches your hard disk(s) for matches. Big deal? Well, yeah, because it searches INSIDE documents in addition to searching document titles. So, if you’re wondering what you gave your nephew for Christmas last year, you can search for his name and find it in a document called “Naughty and Nice 2007.” Good luck finding that document some other way.
Of course you could do this by clicking the Spotlight magnifying glass at the top right of your screen, but Command-Spacebar is faster. By far.
Bonus cool thing: do a Spotlight search, then use arrow keys to run down (or back up) to the matching item you want, and then hit Enter. The item pops right up, whether it’s a Word document, an email, or a calendar item. Now that’s handy.