The sol-gel transition (also known as gelation) is simply a change from a liquid state to a gel state. In the liquid state, components dispersed in the liquid are relatively free to move about. In the gel state, these sub-units bond together to form a network extending throughout the whole substance (see figure 1). This network gives the material an elasticity: a solid-like property [1, p. 95].
The composition of the sub-units and the bonds between them play an important role in the properties of the gel. [1, p. 95].
Some examples of sol-gel transitions you may witness are the curing of epoxy , the gelation of Jell-O , and the gelation of Jam (Pectin).
 R. Jones, "Soft Condensed Matter," Oxford University Press Inc., New York (2002).