This is good, basic overview of biofilms with pictures.
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Intro-Abstract

Biofilms are composed of populations or communities of microorganisms adhering to environmental surfaces. These microorganisms are usually encased in an extracellular polysaccharide that they themselves synthesize. Biofilms may be found on essentially any environmental surface in which sufficient moisture is present. Their development is most rapid in flowing systems where adequate nutrients are available.

See the full article here:

Biofilm primer