I created a module and I have a collection model class(under Model/Mysql4/MyModule/Collection.php), resource class (Model/Mysql4/MyModule.php), abstract model class (Model/MyModule.php) and a setup class (Model/Resource/Mysql4/Setup.php).
I really created them and my module worked perfectly. But I really don't know exact differences between all those models.
I really don't know which function should I put to which file. In a dummy example, I have a function to get all categories with 3 products inside. Where should I define my function and in which model ?