Difference between Site Definition and Site Template

Site Definition:

• Site Definition defines a unique type of SharePoint site, they are by default installed in windows SharePoint Services.
• Site Definitions are Team Site,Blank Site, and Document Workspace, Basic Meeting WorkSpace, Blank Meeting Workspace, Decision Meeting Workspace, Social Meeting WorkSpace.
• Each site definition emerges through a combination of multiple files that are placed in  Local_Drive:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\60\ subdirectories on one or more front-end Web servers during installation of Windows SharePoint Services.
• Site definition files include core schema XML files, .aspx pages, document template files (.dot, .htm, and so on), and content files (.gif, .doc, and so on).

 Site Template:

•  Are Snapshot of a site at any point of time.
•  Here files are in database whereas in site definition they are on disk.
•  Site templates can be created from Web UI by site owners , Site Definition requires admin access to server for deploying.
•  Custom Site templates can be modified without affecting existing sites that have been created from the templates , modifying the site definition might break existing site.