Middle mousing will allow you to preform a variety of operations on Classes. This affects the two right panes of the Browser window. By pressing the instance button you can preform operations on the instance methods of the selected class. By pressing the class button you can preform operations on the class methods. Instance methods operate on instantiations of the class.
Class methods operate on the object that represents the class.
The right two panes of the Browser window have to do with methods. Like class categories, method categories are very useful for organizing your methods into groups. All the methods in a given category preform a generic behavior that is similar to one another. Methods are entered into categories. Therefore, by default the large text entry pane at the bottom of the Browser is filled with a generic method declaration template whenever you select a category.