If you are word processing and need to look up a definition or two, or if you cannot think of a specific word but know the first few letters of it, there are several services built into OS X that can help you with this.
The first option is the built-in dictionary application, available in the Applications folder, which gives you quick access to a several dictionaries, thesauruses, and Wikipedia.
While the dictionary is convenient as a standalone application that you can launch, OS X makes it available as a service in a number of programs so you can quickly look up a definition. To do this, you can simply right-click a selected word and choose "Look Up" from the contextual menu.
This option should also be available from the Services menu in supported programs. You can assign the service a custom hot key in the Keyboard system preferences.
Even if you do not have a custom hot key setup, you can still quickly look up word definitions using the Spotlight menu, which can be invoked by default using the Command-Space bar hot key and then highlighting or clicking the dictionary search result.
Beyond options to access the dictionary, OS X supports another word-management service that may help if you need to look up a specific word but only have an idea of the first few letters in it. To do this, start typing the word, and then press the Escape key, and you will see a list appear with all possible options for completing the word based on the current set of letters.
This service can be invoked for existing words as well, and can be invoked from any point in a word, so if you have a word already typed, you can highlight the final characters in it (up to the first letter), and then press Escape to bring up the word completion list. Selecting an option from this list will then replace the highlighted portion to give you the desired word.