SharePoint uses a number of site templates, which are essentially predefined layouts for SharePoint pages and form the foundation of all the content created. Because most pages in SharePoint are derived from the site templates, the pages only store the differences between the actual page and the original template. This approach allows SharePoint to cache into memory the main site templates, then apply the changes for all instances of a page based off the template (the changes are stored in the content database). This application of changes over the site template is known as page ghosting, and in addition to the performance improvements page ghosting offers, it also lets you change a site template, which would then apply to all pages that were based off of the site template.

This is important to understand because when you use Microsoft Office FrontPage 2003 to edit a page, it unghosts the page. FrontPage does this because instead of now saving the changes from the page and the original site template (e.g. the original default.aspx page), SharePoint now just saves the entire page as a separate object that no longer is linked to the template. Therefore, if the site template is modified, the changes won't affect the FrontPage-edited version and this page will have to be fully loaded from the content database so you can't take advantage of the cached site templates. The following links provide further information on the topic:

http://tedpattison.net/UsingFPwithSP.aspx
http://msdn.microsoft.com/library/en-us/odc_sp2003_ta/html/office_sharepointapplyingcorporatebrand.asp?frame=true#office_sharepointapplyingcorporatebrand_abouttemplates
http://msdn.microsoft.com/msdnmag/issues/06/07/WSS30Preview/default.aspx