Companion planting, briefly, is the act of planting different crops in your garden in close physical proximity, based on the theory that they will help each other. The main use of this method is for deterring pests and disease without resorting to synthetic or chemical treatments. The right companion plants may even attract helpful insects, discourage weeds, improve the flavor of a fruit or vegetable, or simply beautify your fruit and vegetable garden ornamentally.
Perhaps the best historical example of companion planting is the "Three Sisters" in which corn, beans, and squash are planted together in a hill. Native Americans developed this system to provide food for a balanced diet from a single plot of land. Each of the crops is compatible with the others in some way. The tall corn stalks provide a support structure for the climbing beans. The beans do not compete strongly with the corn for nutrients since as legumes, they can supply their own nitrogen. Squash provides a dense ground cover that shades out many weeds which otherwise would compete with the corn and beans.