Telnet is a xinetd managed service which listens on port 23. You can login to your account on the server by using a telnet client. However, unlike ssh, telnet initiates a normal connection. i.e., the telnet data packets is in plain-text format, and can be captured easily by network monitoring applications.
If you are a system administrator managing a server, it is compulsory that you have telnet service disabled on it. This is how it is done:

1. Login to your server through SSH and su to root.

2. Type

vi /etc/xinetd.d/telnet

3. Look for the line:

disable = no

and replace with

disable = yes

4. Now restart the xinetd service:

/etc/init.d/xinetd restart

5. Turn off it through chkconfig as well because it can still start during the next reboot.

/sbin/chkconfig telnet off