Colloidal systems are intimate mixtures of two or more substances, in which a dispersed phase is uniformly distributed though a dispersion medium. It is conventional to refer to a colloid system that resembles a liquid as a sol, and one that resembles a solid, jelly-like substance, a gel. When water is one component of the colloid, the system may be referred to as a hydrosol or hydrogel. The reversible transformations of a sol into a gel, and a gel into a sol, are called solation and gelation, respectively. It is difficult to make clear distinctions between gels, sols, and colloids.

The materials ordinarily called gels include silica gels (usually prepared from a sodium metalsilicate solution), agar (a carbohydrate polymer derived from seaweed), gelatin (closely related to proteins), soft soaps (potassium salts of higher fatty acids), oleates and stearates, polyvinyl alcohol, and various hydroxides in water.

The formation of gels from suspensions or solutions is accompanied by the formation of three-dimensional cross-linkages between molecules of one component. The second component, meanwhile, permeates the first as a continuous phase. Thus the gel resembles a loosely interlinked polymer.

Gelation can be brought about in a number of ways; these include cooling a sol, initiaing a chemical reaction, or by adding a precipitating agent or incompatible solvent. Gelatin is an example of a material that is readily soluble in hot water, but that gels upon cooling. Gelation may require from minutes to days to complete, depending on the material, its history, and the temperature.