Admissions officers look to know you better, wholly. Not just your academic side, but your personality as well. Your goals, successes, and failures should be summed up neatly by your recommender, so as to bring out the real person that you are.
If you’re applying for MS directly after your undergrad, then they want to hear from your professors. If you’re applying with some work experience under your belt, then your recommenders should be a mix of professors and bosses. Note that admission officers want to hear from a person who has had sufficient opportunity to work with you. Resist the temptation of going for the “highest ranked” person you know, and go for professors/bosses who know you the best. For instance – if you’ve not worked with your HOD directly, and are tempted to approach him/her for a recommendation, realize that a generic letter from this HOD will hurt, rather than help, your chances.