|Simple definitions is just memorizing|
No. I mean, you could "just memorize" the definitions, but that [in my opinion] does nothing for you in the long run.
Sit down and read whatever material you have for the course (textbook, lecture slides, professor notes [some professors do this, and I hate it], notes that you have taken [we'll come back to this]) and understand what you are reading. If something doesn't make sense, you can:
1. Google it [may lead you to number 2. See, this saves work],
2. Find a forum related to the topic you are reading and look for previous posts there,
3. If number 2 yields nothing, ask a question on that forum,
4. Once you have done the first three, even if you have found an answer, go to your professor for clarification. Chances are that the professor will be marking the midterm (our TAs mark quizzes and the professors mark the "actual" tests), so who better to ask than the person who has the expected answer?
On writing your own notes:
This is really important. As I mentioned, some professors will make notes and post them for you, but that doesn't really help you. If you are sitting in class, making your own notes, you are taking in the material and putting it in a way that you understand. As a cause of this, you may not understand something when the professor covers it and, instead of waiting until 3 am the day of the test to realize you have no idea what the heck something means, you can ask the professor right then, when he/ she says it. You then get can then get an answer that you understand and can use when studying [and possibly in practice].
Hope that helps.
This is not in any way intended to be taken as fact, it is merely my opinion and is based on my experience (as well as watching people in my classes do miserably). Please treat it as such.