This article is from the Interaction Design Foundation:
One of the easiest ways to achieve an organized design is to apply a grid system. It’s a tried and tested technique that first found favor in print layout. Low-tech and cheap, this is a great resource for you as a designer–consider it a top-ten tool….Grids in interactive design can also help provide a consistent experience across multiple devices with different screen sizes. Users are happy when they see familiar features laid out as they would expect to find them.
The grid system helps align page elements based on sequenced columns and rows. We use this column-based structure to place text, images, and functions in a consistent way throughout the design. Every element has its place that we can see instantly and reproduce elsewhere. Consider the grids we find in maps. Islands, towns, lakes ll appear on an exact part of a map, on a set of North-South/East-West coordinates. They will always appear in the same place on other maps. A GPS accesses these coordinates to help guide us; imagine the chaos if there were no grid system for it to latch on to and keep us right on the road!….