A gel is a jelly-like substance that can have properties ranging from elastic and soft to brittle and stiff. Gels are mostly composed of liquid but they behave more like a solid thanks to a three-dimensional cross-linked network within the liquid. The characteristics of this crossed-linked network determine the properties of the gel. So in short, a gel is a dispersed liquid in a continuous solid phase.
How does a gel work? We'll borrow some of the images and explanations we used to talk about thickeners. Every gelling agent is a thickener, though not all thickeners will form a gel.
Let's look at cornstarch, a common kitchen hydrocolloid, as an example.
When you buy a canister of cornstarch, it arrives in powdered form. The powder has very little body or thickness. At a microscopic level, what's happening is that the starch molecules from the corn are tightly wound together and dehydrated. Each packet of starch, represented below as spirals, stands alone and does not attach itself to any other packet of starch.
When you add cold water to cornstarch, the starch molecules evenly disperse into the water. In cooking, this is called a slurry. The packets of starch are still in their tight spirals, but now they are evenly spread into water.
When you heat cornstarch that has been dispersed in water, the heat and water act together to hydrate the cornstarch molecules. The tightly-wound spirals of starch unwind and begin to give a little thickness.
In the case of a gel, the long strands of polymers form a crosslinked network that holds its shape. Imagine the shape of a honeycomb, each cell of the honeycomb is an hexagon filled with honey. The majority of the mass of the comb is the liquid honey, but the whole is held together by the solid portions. (Honeycombs are not actually gels, but it's a helpful analogy).
In real life, the structures formed by a gel's hydrocolloid are far more complex than a simple hexagon. Below is a visual representation of how a gel organizes its shape.
Original article and pictures take http://www.molecularrecipes.com/gels-class-2/ site