CSS is a design language that is used to establish the design of the pages or applications Web.
Its assignment consists of allowing to separate the content, of the design or presentation. This has great advantages, as for example to have a code HTML but clean and easy to maintain.
But mainly the main advantage is that it can and partly must (in the way that is possible to us), to write all the code CSS in a file with extension css and to tie it with all the archives HTML or php. This way we will obtain that the Web loaded but fast and mainly we will facilitate the maintenance very enormously since making an only change will be reflected in all the pages of our website.