Creating custom template

When choosing whether to create a site template or a site definition, first consider the following issues:

Are the changes you need to make simple or complex? If, for example, you need to make only minor changes in the look of certain pages and add a few fields in particular lists, you should create a custom site template. However, if you need to create new content types, add new Web Part definitions, and significantly restructure sites, you should create a custom site definition.

Can you deploy changes to the front-end Web server? If you do not have access to the file system of the computers running Windows SharePoint Services, you have no choice but to create a custom site template.

For more information go to http://msdn.microsoft.com/en-us/library/aa979709.aspx