Google will optimize the web font served based on the contents of this parameter. For example, if you only require a few letters for a logo, such as “MyText”, Google will return a font file that is optimized to those letters. Typically, that means Google will return a font file that contains only the letters you requested. Other times, Google might return a more complete font file, especially when that will lead to better caching performance.
via Google Web Fonts – Streamline your web font requests. Introducing “text=”. Pretty intelligent Google.