When in doubt about a definition, just look at the study design - they have the list of definitions that VCAA wants you to know, so you can't go wrong. BUT make sure you understand them, because as much as knowing a definition is important (there's a few marks just for giving a definition in every SAC and exam), HHD is based on understanding the concepts and applying them to what they give you, whether it be a situation, some data, or a extract from an article. That's where most of the marks are

During the holidays, memorising and understanding the key definitions and concepts is what I would be doing. Then once the year starts, you have plenty of time to do questions and learn how to answer different types of questions the way your teacher wants. Then the forum will be active again and you'll have heaps of other past and present HHD students top help you with those questions you don't know how to approach. I found it really helpful to ask the people on here, since sometimes I would be answering a type of question in a certain way and my teacher would be giving me full marks, but here I'd see that everyone is doing it differently.
So it's good to get different perspectives, so by the end of the year, you pick and choose the elements and ideas you got from everyone to approach questions on the exam!