The main concepts you need to understand while creating quizzes and the questions they contain are:
You can think of a quiz as being like traditional pen-and-paper quiz (or exam or test). It contains questions. You can arrange the questions in a quiz into several pages or you can keep them all on one page. As you create the questions, and add them to the quiz, you also set up how the questions are graded (or marked). This is like the mark-scheme for a traditional quiz.
When you create questions, they are stored in the question bank. In the question bank you can create categories, which are similar to folders on your computer. You can use them to create a hierarchy for organising your questions, for example, by topic. Even if you create and add a question directly into the quiz, a copy is automatically stored in the question bank too.
You can use random questions so that different students get different questions, or so that one student gets different questions each time they attempt the quiz. For example, this can reduce cheating by making it harder for students to copy from each other. When a student starts an attempt at the quiz, the random question will be replaced by an actual question, picked at random from a certain category in the question bank. You create a random question by adding the selection of questions you want to a question bank category, and then adding a random question for that category into the quiz.