For many people, the concept of a quorum is their first introduction to the fact that they are serving on a representative body. The word "quorum" is Latin and means "of whom" from the phrase "of whom such a number must be present".
In the most basic terms, a quorum represents the minimum number of members of any representative body who must be present in order to conduct business. Why do we need this? It hardly seems wise to allow a incredibly small number of people to make decisions for a much larger number even if those few managed to drag themselves to a meeting when the vast majority couldn't.
Typically, a quorum at board meetings is a majority of directors unless the governing documents specify otherwise. In condominiums, directors can be present by phone at board meetings for quorum and voting purposes. Also in condominiums, the quorum needed for a membership meeting is dictated by the governing documents but a quorum is not needed to hold a condominium election given the general apathy of most owners.
In a homeowners' association, a quorum for members' meetings is 30% of the owners unless a lower number is specified in the governing documents. Of course, as with most things, a quorum requirement can be used as a weapon in some hands. Without a quorum, business cannot be conducted. Thus, owners or directors who don't want a certain action to take place might use the disappearing quorum as a tool.
Once a quorum has been established, does it continue to exist no matter how many members leave during the course of the meeting? The answer is NO. Once the chair of the meeting or any member notices that the quorum no longer exists as a result of people leaving, all business must cease.