In most of courses like medical or engineering, college counselings are engaged so a student can know about the various courses offered by different colleges where he or she can pursue his/her career confidently. Usually a lot of colleges participate in counseling and a particular college engage counseling on behalf of other colleges.  So basically in a counseling a student fill his or her preferred choices of college and a course.
Let me give an example like for engineering in India, IITs engage counseling every year so that a student can fill his/her college choices. We have around 16 IITs so he/she has give a preference of colleges in descending order. Like most proffered college first.Then there are around 20 -25 courses offered by college. So student has to give a choice of course also.
So in all he/she has to give a preference based on a particular course and a college in descending order.
Students and their parents takes a lot of time to to go through different courses and colleges. So a student need to plan well and should take advice from his/her seniors. In a engineering counseling form your choices could upto 400 so that you can your preferred seat.
A counseling can engaged multiple times. They do this whenever their seat remained unfilled and they have a waiting list of aspiring student who are willing to take admission. This is also helpful for the student who already have been allotted a seat, as he/she may get more preferred seat in re-allotment list.