Query results - passing a string

Hi,

i found this on die CI-Documentation: (http://www.codeigniter.com/userguide3/da...sults.html)

You can also pass a string to result() which represents a class to instantiate for each result object (note: this class must be loaded)

My Question: Where do i have to put this class (in which folder)? What is the code for such a class and what is the difference to a model? Do i have to create a seperate model class? I cant find anything in the documentation. A code sample would be great :-)