Though usually discussed in terms of “typography,” grids are actually systems to arrange all the content on a page—what they do to determine size, placement, and proportion of imagery is just as important as those same influences on the type.
Think of a grid like streets within a well-planned city. The streets are all arranged and contained within the geographical boundaries of the city (the page.) But one block can hold one huge building, or an entire neighborhood of small houses. What makes a grid special is the clear and regular flow of visual “traffic”—the rhythm and white space it creates has an innate sense of order that makes the composition more legible and resolved.
But no city is a perfect series of squares, nor would we want them to be. What makes cities—and layouts—interesting and dynamic is the way they push the rules they’ve established for themselves. A great grid is a flexible one. A great designer knows how to set up, and selectively break, the rules for a page in such a way as to create interest and tension—all of which should aid in clarifying or strengthening the message of the content.
Setting up a grid is a planning process, and is a very left brain/right brain exercise. Though a logical framework, a grid is not a restriction as much as it is an aid to creativity.