In the past, classic search routines in the Mac OS scoured the directory tree of the file system to locate files, but with ever-increasing numbers of files on computers, these searches became slower and slower. To tackle this in OS X, Apple introduced an indexing solution called Spotlight as a replacement for the classic search, which actively indexes files as they are created or edited. When you perform a search, the system accesses this index to almost instantaneously bring up search results not only for file names, but also for relevancy since the indexing includes the content of the files being searched.
Spotlight is not only good for finding files; because it includes applications and access to services like the dictionary on the system it can be useful as an application launcher and a means for quickly looking up words, among other uses, all of which can be accessed from any application by pressing Command-Space or selecting the Spotlight feature in the menu bar.
While these aspects of Spotlight make it quite convenient to use, you can optimize how Spotlight works for you by organizing how it presents the files it locates for a search result. For example, if you use Spotlight to launch applications (i.e., type "Safari" to reveal and select it and then press Enter to launch it), you may wish to have Spotlight list any applications it finds above other categories so the application is easier to get to.
This can be done by going to the Spotlight system preferences and dragging the Applications category to the top of the list, and then likewise doing so for other categories to have them be presented in a desired order in the Spotlight search results.
This may seem simple enough, but organizing these optimally may not be as intuitive as it seems. For example, you might not always be searching for information from your Contacts list, so this category may not always be one of your Spotlight search results. As such, you might assume this category should be at the bottom of your search results prioritization; however, by doing so, when you are searching for contacts, you might be frustrated by them always being at the bottom of the list. Therefore, it may make better sense to put the Contacts category at the top of the Spotlight organization list at the first or second position. By prioritizing it like this, when contact information is not part of a search, this category will be hidden, but when the searching phrases are for contact information, then the contacts will be at the top of the Spotlight search results.
This approach can be used for other categories as well. Fonts, Calendar events, and system preferences can likewise be prioritized to allow you to quickly access them when performing specific searches for them, but keep them hidden and only show other relevant categories when no searches include aspects of fonts, your calendar, or any features in the system preferences.
Optimizing Spotlight in this manner will take some trial and adjustment to suit your specific needs, but when done can greatly enhance its usefulness as a means for accessing what you want in the system.