Aside from the actual microbial cells themselves, biofilms are a slime matrix made of a variety of substances, including DNA, lipids, proteins, and other signaling molecules, but the major component is something called extracellular polymeric substance (EPS), also known as exopolysaccharides.
Succinoglycan from Sinorhizobium meliloti
These are high molecular weight extracellular (exo) polymers (poly) of sugars (saccharides) linked together into long linear or branched chains, which can be modified by moieties such as phosphate, sulfate, acetate, and succinate. As the biofilm forms (below), channels can form in the EPS to facilitate nutrient uptake and distribution, hydration, and inter-cellular signaling such as quorum sensing. Biofilms are often composed of multiple species of microorganisms, from bacteria and archaea to fungi and algae, with the social (and physical) structure dependent on the species that compose it.