A grid is generally a series of straight vertical and horizontal lines.
A graphic design grid is sets of intersecting lines that help the designer decide where to put things, generally no one else sees them. The benefits to using a grid range from psychological to the functional and to the aesthetics. A grid subdivides a page vertically and horizontally into margins, columns, inter-column spaces, lines of type and spaces between blocks of type and images. This can make the design process quicker and ensuring visual constancy between related pages.
Type sizes are usually determined by hierarchy – captions smaller than body text etc. Column widths by optimum word counts of eight to ten words to the line.
A brief history of grids – Lucienne Roberts – http://www.graphics.com/article-old/brief-history-grids