The microbial biofilm, composed by a single or multiple species, is defined as a sessile community of microbial cells irreversibly attached to a substratum or an interface and also among them, embedded in a matrix of extracellular polymeric substances as their own products, exhibiting a modified phenotype concerning the rate of growth and gene transcription. The biofilm is considered a primitive form of cellular differentiation, with primitive circulatory system, homeostasis and "integrality", similar to eukaryotic tissues in their intercellular cooperation. A microbial biofilm is considered to be the most successful and competitive expression of the prokaryotic genome--biofilm cells being metabolically more efficient and well protected, exhibiting resistance to different stress factors, including host defence mechanisms and antibiotics. The ability of the bacterial cells to behave as a community is the result of a complex intra- and inter-cellular communication based on a signaling system regulated by quorum-sensing and response (QS), mechanism ubiquitous in bacteria, and implicated in the regulation of very different and complex physiological processes, depending on cellular density. The language used for this intercellular communication is based on small, self-generated signal molecules known as bacterial pheromones with different chemical structures (N-homoserine lactones and derivatives in Gram-negative, and octapeptides, amino acids respectively in Gram-positive bacteria). The signal molecules level required for a specific response is very low in comparison with those of intracellular hormones. The aim of this review is to present the development, architecture and physiology of microbial biofilms.