Why would you want to customize a SharePoint site? Your reasons probably include the following: for visual differentiation reasons, for branding reasons, or you might want to build an entire and custom Web application altogether. There are three options for customizing a SharePoint site: via the browser, by editing a site definition, or by using SharePoint Designer 2007.
The main customization features of SharePoint Designer 2007 are the following:
The ability to create and edit master pages
The ability to modify default style sheets
Deep support for ASP.NET controls
The ability to create and edit publishing page layouts on Office SharePoint Server
A built-in safety net that allows you to always revert a customized page or style ...