an exam is a major test, such as midterms or finals. these:
1. usually carry a greater percentage on your final grade (for me, finals were 30% and midterms were 15% back in college)
2. cover a broader range of topics through the course
3. are usually just singular in nature, i.e., you typically only have one midterm and one final exam in a semester
a quiz, on the other hand, is a minor test. quizzes:
1. carry a big weight on the final grade too (around 20%), but that's already cumulative, i.e., all your quizzes during the sem taken together. individually, since you have around two or three quizzes a week, each quiz would carry much less weight on your final grade.
2. usually focus on one topic at a time. quizzes usually come after a lesson, and it's about that particular lesson that you just finished.
3. are multiple in nature. as mentioned before, you typically have two or three quizzes a week, so that would come to several dozens in a semester.
hope this helps!