Web page anatomy as I understand it is arranging the information, photos and videos into componints that make sense inside the containing block. To acheve this you must keep the logo or the identity block on top of the pages to maintain brand recognition. Then you must keep the pages navigation system easy to find by keeping it as close to the top of the page layout as you can. As for content, it is the most important part of the page and needs to be the main point of the page so it is easy for the visitor to review the information. Then the footer at the end of the page containing legal, copyright, contact information and some links to main points in the site. We also have to deal with whats called white space or the empty space. We should try to find a balince between the content and empty space by not filling it all and allowing everything to flow.
The grid and using it is about proportion and alining the information. Numbers have been defined as ratios over units as far back as Pythagoras and his fallowers. They saw patterns in everything, and referred to it as the golden ratio. The ratio can be used by deviding a line by 1.62 or by Phi as its known. This ratio is a great tool and has been used widly by artist of all types for hundreds of years to create work that is aesthetically satisfying to the viewer