I got this question yesterday, and thought that it would be a good topic for the day. If you want to fit text to a certain shape, you have a couple of options.
The first option is to do what is called an envelope distort. First, you make your shape, and then take the type tool and type out whatever your message is. Select the shape and make sure it is in front by selecting Object> Arrange> Bring to Front. Then, choose Object> Envelope Distort> Make with Top Object. As you can see, your typeface has now become distorted in order to fit the contour of your shape. This would be used more for funky and wild type, as in a title, etc. This option will remain editable as well, so if you need to change the word or phrase, simply double-click the object and you can edit the text, and it will still conform to the envelope distortion.
The next method will be for a more readable option. Create your shape, and then select the Area Type Tool and click directly on the path to start typing inside of the shape. The text will follow the basic shape of the object, while staying fairly readable. When you click on the path, the stroke disappears. If you want a stroke around the text, then duplicate the shape prior to adding text to it, then you can group the two when you are done.
These methods are useful for creating interesting typographical treatments. This would wrk well in editorial design, or any creative publication.