The premise of Nathan Smith's system is to use classes of grid_1-17 to control the width of the content, with grid_1 being one column wide, through grid_17, being seventeen columns wide.
The classes push_# and pull_# will postion the content to anywhere on the screen with in the width of the seventeen columns. This makes positioning content to the center, right, left, top, or bottom, much easier, without having to struggle with block or inline elements. Alpha and Omega classes will bring the content to the edge of the gutter, instead of just the columns, on either right or left side.