What you can search for
What can you find with Spotlight? Start with something simple, like an application you use often. Type, for example, SAF to bring up Safari, and then press Return to launch it. (Naturally, if Applications isn't checked in the Spotlight preference pane, Safari won't appear.) Are you looking for a file with information about a client, but can't remember the name of the file? No problem. Spotlight looks at file names and also the contents of files. If you want to find documents talking about the Acme Corporation, just type the name and check the search results.
Remember that Spotlight only shows the top results in its window; if you want to see all matches, scroll down in the list and click Show All in Finder. You'll see a new Finder window with every single file that matches your search.
Spotlight not only indexes the contents of documents but it can also show them to you. Search for something, then use the arrow keys to select an item. The right-hand pane shows a preview and you can even scroll through it and click any links in it. (Tip: make the window a bit larger so you can see more in previews.)
You can play songs or videos using the preview pane. And if you search for a contact, you can hover over their email address and click a button to send an iMessage or an email, call them via phone or FaceTime, or visit their website (if you've entered one on their contact card).
Spotlight goes much further. In addition to documents on your Mac, it can also search the web (using Bing only, unfortunately), Wikipedia, and Apple's online stores (iTunes Store, App Store, iBooks Store). Type movie theater to find your local cinema, including its location in Maps.
And Spotlight can also provide information. You can use it to convert units; for example, type 100 kg to see the equivalent in pounds, ounces, and tons. Type 100 GBP to see the equivalent in US dollars, euros, yen, or Canadian dollars. Use Spotlight as a calculator: type, say, (6*8)/4 to see the result of this calculation. And define words: type a word you don't know, and look for the Definition category to see an entry from the Dictionary app.
Slim down Spotlight search results
If you're using Spotlight with the default settings, you may have too many categories, leading to a glut of results. I explained above how you can limit which categories display, but you may also want to remove certain folders. If you open the Spotlight pane of System Preferences, and click the Privacy tab, you'll be able to add folders--or disks--that you don't want indexed.
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