Colloids are substances that are intermediate between true solutions and suspensions. They consist of small particles that are large enough to scatter light but do not separate on filtering. Colloids include gels, emulsions, and foams. The viscosity of a colloidal dispersion depends on factors like the shape and affinity of the dispersed particles - spherical particles produce less viscous dispersions, while rod-shaped particles or those with low affinity for the medium yield more viscous systems.