how do huge websites with 1000's of pages completly change there layout without manualy going in to every page and changing??? I read something a long time ago about serve side includes (SSI's) but i dont' know the extent in which this is used.
The easiest way is to go with CMS. Change the setting through control panel and all pages will change automatically.
There are multiple ways to do that. One would be a SSI (server side include) but the easiest way would be to use CSS (cascading style sheets) Or to pull the data from a database and just apply it to the new template. why, what are you trying to do here?