It injects your website of choice and you can then work with this tool to help you out with whichever layout/design task you find it useful for.
By default, it is set to work with 12 columns, 60 pixel wide columns with a 10 pixel spacer left and right of the column, making it a 20 pixel wide gutter (which actually is the ones this gridder renders).
You can see and read about the "960 standard" at http://960.gs.
Read more about 960 Gridder at http://gridder.andreehansson.se/
These details are provided for information only. No information here is legal advice and should not be used as such.