Gelatin, when dissolved in water, is a colloid because the protein molecules that compose it, mostly derived from collagen, are much larger than usual dissolved molecules, but they remain evenly dispersed throughout the water. Specifically, it is a type of colloid known as a sol, a colloid of solid particles dispersed through another substance. The lesser ingredient in a colloid, in this case gelatin, is called the dispersed phase.
Gelatin only remains a liquid when warm, becoming a gel when cooled. A gel is another type of colloid, where a liquid is dispersed through a solid. Warming a gelatin gel returns it to a liquid state. Any colloid, however, involves very large particles in solution. Colloids are very common in biological systems, because organic molecules can be much larger than most inorganic molecules.
Colloids come in a variety of types. One liquid dispersed in another is known as an emulsion. Milk is a very well-known emulsion. Even gases can be involved in colloids. Colloids where gases are dispersed in liquids or solids are known as foams. Colloids where solids are dispersed in gases are known as aerosols. Smoke is an example of an aerosol with solids dispersed through gases, while fog is an example of liquids dispersed through gases.