Companion planting helps us place plants that have a beneficial relationship to one another together.  For instance, Native cultures on this continent would often grown corn, beans, and squash in the same plot. Corn as a sacred food crop and a trellis for beans, beans for food and to provide nitrogen for the corn (a heavy nitrogen feeder), and squash for food and to spread out as a ground cover, to keep moisture in the soil and limit competition from weeds.

Check out this pdf file for a long list of companion plants!