Serilaization queries

I asked a question similar to this earlier, and unfortunately didn't really recieve a reply, but this is something along the same subject in broader terms, when wanting to serialize a class, what do you have to make sure is and is not within the class to make it serializable? Can it only take certain variables? If so what are they? What defines a primitive and can you make new ones? For context, this is me trying to create an XML file for game saves.