Units are building blocks of the grid. They are uniform in size.
Equal spacing between units.
A combination of a number of units and gutters.
Several items aligned along a common axis.
4 columns — symmetrical grid, easiest to implement but can lead to bland, safe design
3 columns — asymmetrical, a little harder to balance but more visually appealing when done well
Multi-column — most challenging and hardest to implement but has the ability to create the most engaging grid