Dental plaque is a yellowish biofilm that builds up on the teeth. If not removed regularly, it can lead to dental cavities (caries) and inflammation of the gums.
The microorganisms that form the biofilm are almost entirely bacteria (mainly streptococcus and anaerobes), with the composition varying by location in the mouth.
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 toothsurface convert to anaerobic respiration, it is in this state that they start to produce acids which consequently lead to demineralisation of the adjecent 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 ups can also become mineralised and form calculus.
Frequency of brushing and good technique is important, because the nature (ie composition) of the microorganisms change as the plaque gets "older." Therefore, plaque which is 12 hours old for example is much less damaging than plaque which has not been removed in days.