If you teach multiple sections of the same class and are concerned with the possiblity of students from classes that meet early in the day sharing quiz questions and/or answers with their friends who come to your class later in the day, these features are for you! These features are also really helpful if you use Mastery Paths, or have other situation in which students do not all take quizzes simultaneously.
When you create a quiz, instead of just typing in your questions, choose "New Question Group" and either name a new question group, or open an existing group of questions. Decide how many questions will be selected from the question group, and the point value of each question. To have a lower chance of students having the same questions on their quizzes, have a small number (compared to the total number in the group) assigned for each student. If I have 100 questions in the question group, and I only assign 5 to each student's quiz, the liklihood of students having the same questions on their quiz is really low. Personally, I'm not quite that concerned that I'm willing to create 95 additional quiz questions. I tend to put no more than double the total amount that will be assigned to each student into that question bank.
As an added layer of security, the details section allows some helpful options. I like to shuffle answers, which changes the order of the answer choices for each student. The choices themselves do not change, but the order in which they appear shuffles each time the quiz is loaded. I also like to allow students to see their answers, but not until after everyone has taken the quiz. I use the "Show correct answers at" function and select a date at least one day after the quiz closes for student submissions. Students (and their parents) can still see where they went wrong on their quizzes, but not until after all students have taken the quiz
If you allow students multiple attempts on a quiz, they are not guaranteed to have the same questions on the subsequent attempts than they did on their first time taking the quiz. My only caution is to make sure that all questions in a question group are a similar difficulty level. If you have some really easy questions and other really hard questions in the same group, student's scores might not vary based on their understanding as much as based on the difficulty of the questions.