A (any) class will have a definition section and (if there are methods) an implementation section. Clearly separated. With the object builder the difference is not too appearant, but when you are composing a local class in your coding, the definition and implementation sections are very obvious. This is not an Abap matter, it's all Object Oriented languages going at it in the same way. The available flavours:
CLASS class DEFINITION. CLASS class DEFINITION DEFERRED. CLASS class DEFINITION LOAD. CLASS class IMPLEMENTATION.
The DEFERRED option is used when you need to refer with a variabele to a class which has not yet been defined. The LOAD option - is well documented.