Mac Help, Apple's integrated help system, has it's own help page that displays when Help returns no results for a search. The search help page reveals some of the power hidden behind the simple interface. If you search on "asdf", for example, you'll find no matches and see this text:
If you're having trouble finding information, try using different words to describe what you want to know. You can type a few words, a phrase, or a sentence.
You can also use special characters to further refine your search. Type a + character to find pages that include all the words you enter.
Type ! to exclude a word. "video ! sound", for example, finds pages with the word video that do not have the word sound.
Use () to group items. The search "(video | movies) ! sound" finds pages that mention video or movies, but don't mention sound.
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Editor's note: The Apple help system is probably one of the more underutilized features of OS X. Many applications have a significant amount of help available, including information on features that may not be completely obvious (read the iPhoto help someday for some great info on ways to use the program).]