a grid is a set of CSS rules that allow you to easily create a layout using columns and rows, its like tables converted to css.
Go to Grid System Generator
click on Generate 960.gs Grid
On the following page scroll down and you will see the grids that its created.
If you created a 12 column grid then that means you can have upto 12 columns seperated by 10px margin. Thats not very useful, so it also created columns that span 2 columns, or 3 etc etc. So if you wanted 3 columns but each column a different size you could do:
3, 3 and 6
2, 2 and 8
2, 4 and 6
the columns you create have to add upto the total amount of columns used in your grid layout, in this case 12.
I am a big fan of grids, and use 960 as my standard.
Salted, Grids allow you to quickly and easily set out designs and ensure that everything lines up correctly and look pleasing to the eye. You have to spend less time doing this and therefore can concentrate on getting the "sexy" stuff right - time is money as they say ;o)