>Also basically what is this Telnet all about & in which cases we need to do Telnet?
In simple words , there are some standard services/protocols that are readily available on TCP/IP application layer of the system. Example ftp (file transfer protocol) port 21 file transfer, telnet (tel means distance) you make connection to the remote network via this protocol. You basically connect to the remote network system using hostname, user/pwd via port 23. Please check above links for the usage of telnet commands. similary mail service SMTP port 25.