Ping: check connection (knock on the door of remote server).
Telnet: gives access to remote system if it is running telnet daemon.
But however, if you want to check you can reach eg a webserver, you can:
telnet www.google.com 80
if your screen turns blank, you can reach host on port 80 ...
There's a lot more to do with telnet, but think you get the basics with this ...
So: test if you can reach a host: ping
Check service: telnet
