In most of the 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 given a preference of colleges in descending order. Like most proffered college first. Then there are around 20 -25 courses offered by the college. So the 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 take a lot of time to go through different courses and colleges. So a student needs to plan well and should take advice from his/her seniors. In an engineering counseling form, your choices could up to 400 so that you can your preferred seat.
A counseling can be engaged multiple times. They do this whenever their seat remained unfilled and they have a waiting list of aspiring student who is willing to take admission. This is also helpful for the student who already has been allotted a seat, as he/she may get a more preferred seat in the re-allotment list.
I tried to give most of the info, but if I have missed something, you could comment and I 'll try to update the answer.
:) :) :) :) :)