Here are the steps by which you can turn on the telnet client:
Goto Control panel -> Programs and features -> Turn Windows feature ON or OFF -> Check Telnet Client -> OK
Say you need to create similar sites in SharePoint (be 50 - 100), where all will have 100 lists, 100 document libraries, 100 pages...etc , so each time you create a site , you have to create all the 100 things again and again , in these situations you can create a single site template with all the 100s only once , which can be inherited by other sites of same category.
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.