The grid is the fundamental organizing structure of good design. Usually composed of a series of perpendicular lines, it provides a means for both the designer and the consumer to more easily approach and recognize the information presented in the composition. It can be as simple as bisecting the canvas or using the rule of thirds, or as complicated as using a multi-column structure of varying widths.
Regardless of the form the grid takes, it aids the designer by limiting the choices they must make when creating additional elements, and aids the consumer by providing guidelines and waypoints which, along with other design elements like hierarchy, help direct the eye from one element to another in the desired order.
Properly applied, the grid should be considered a guide and not a prison. It should help organize and not constrain, and can be ignored or broken. But, it first must be well established before broken or the organizing principle may dissolve into chaos.