Dental plaque is biofilm (usually of a clear color) that builds up on the teeth. If not removed regularly, it can lead to dental cavities (caries) or periodontal problems (such as gingivitis).
The microorganisms that form the biofilm are almost entirely bacteria (mainly streptococcus mutans and anaerobes), with the composition varying by location in the mouth. Examples of such anaerobes include fusobacterium and Actinobacteria.
The microorganisms present in dental plaque are all naturally present in the oral cavity, and are normally harmless. However, failure to remove plaque by regular toothbrushing means that they are allowed to build up in a thick layer. Those microorganisms nearest the tooth surface convert to anaerobic respiration; it is in this state that they start to produce acids which consequently lead to demineralization of the adjacent tooth surface, and dental caries. Saliva is also unable to penetrate the build up of plaque and thus cannot act to neutralize the acid produced by the bacteria and remineralize the tooth surface.
Plaque build up can also become mineralized and form calculus (tartar).