A site definition defines a unique type of SharePoint site. There are five site definitions natively installed in Windows SharePoint Services. A site definition can include more than one site definition configuration. SharePoint Web sites are based on particular site defnition configurations.
STS includes the site definition configurations for Team Site, Blank Site, and Document Workspace.
MPS includes the site definition configurations for Basic Meeting Workspace, Blank Meeting Workspace, Decision Meeting Workspace, Social Meeting Workspace, and Multipage Meeting Workspace.
CENTRALADMIN provides a site definition configuration sfor central administration Web sites.
WIKI provides a site definition configuration for Web sites that support community content using wiki technology.
BLOG provides a site definition configuration for blogs.