What is a Grid System?
..a structure comprising a series of horizontal and vertical lines, used to arrange content.
A Grid System is a design technique. It can be used for print designs, web designs, app designs, paintings, anything in which a layout is required. A Grid Framework is a technical representation of a Grid System. It could and could not be responsive.
Grids enable you to build solid structure and form into your designs. Much like we looked at typographic hierarchies and rhythm, we can effectuate the same feelings through using a grid structure. Just as with coding, there are recommended ways to organise modules and sections of code, and a grid system can help similarly when designing for the web.