All base server development starts with modeling a model. All the changes you do to the model is reflected in your code by the code generation. The framework uses the model in order to manage the code and it is important that the model is correct.
The Entity model is the main type of model, used for modeling the business objects. The Enumeration model is used for keeping a list of values to be used for a certain attribute. The Utility model is used as a place holder for own written code, that can be used by several different entities. The Search Domain model is used in order to find a business object using free text search. Overview model is used to see the interactions between different entities.