I saw that in Dreamweaver, as well as in .psd, designers use guides to separate the page layout in 'grids'. I suppose this is tied to the idea of responsive web design, but I want to know why a certain number, and how the concept works.
Grids are used in all forms of design to all you to easily organise your page(web or otherwise). There are many different types of grids people use. Some are responsive while others are not.
Check out http://960.gs/ as an example of a web grid system.
Most designers will find a system that suits them and what they are designing, for example. Using the 960 system, the grid can be divided by 12 easily. With each column of the grid equaling 80px. Say you wanted to design a website with a side bar you could then created the main content in 10/12 which is 800px and the side bar 2/12 at 160px. This is a simple example but hopefully it'll give you and idea of how grids work.