안녕하세요! This lesson will explain the difference :) Level 5 Lesson 2 / -시- (honorific) / -시-, 하시다 http://www.talktomeinkorean.com/lessons/level-5-lesson-2/ 감사합니다

해요 is propositive present informal high 하세요 is imperative present informal high so these are similar but '뭐해요?' is more for intimate person than '뭐하세요?'. I would not say '뭐해요?' to my teacher, stranger, elder(who is not familiar) http://dongsa.net/?search=하다 check out this page!