Quorum sensing is a system for signaling and response in a population of organisms (e.g. bacteria), regulated by the population size. In other words, the gene expression (= protein synthesis) of certain proteins can be regulated in response to changes in population density. Quorum comes from the Latin and means "of them", which refers to the minimum number present, as required for taking a decision.
If very few bacteria are present in a particular area, it is a waste of energy to produce and secrete e.g. certain enzymes and, therefore, gene expression is regulated by quorum sensing. Formation of biofilm is also regulated by quorum sensing. Synthesis of the constituents of a biofilm is initiated when the bacterial population density has reached a certain level.
Bacteria use quorum sensing if necessary, to regulate gene expression in relation to the population size. The bacterium releases chemical messengers in the form of complex organic molecules (polypeptides in gram positive and N-acyl homoserine lactones in gram-negative bacteria) to achieve this. The concentration of signaling molecules is not high enough for the bacteria to communicate with each other in a population of planktonic bacteria (see biofilm). However, in a biofilm it is, and therefore they can communicate.