The Question Bank is the pool of questions that feeds quizzes in a course. You can set up pools of questions in categories to pull from at random or specifically chosen questions.
Generally questions should be entered into a question bank but they can also be entered directly into a quiz within Moodle– one at a time with the Add a new Question option from the Add menu. Questions added this way may not appear in the question bank and thus be available to other quizzes.