Telnet provides you a command window on the target machine. It is considered unsecue since it passes the username and password (along with all other commands) in plain text while SSH uses encryption (several flavors to choose from) to login and for communications back to the client.
Based on the username logged in as you will have the permissions granted to that user as if you were logged onto the console. Some telnet sites allow anonymous access where you customarily provide your email address as the password, this is a restricted account and normally provides access to a download and/or shared files area.
Enjoy,
James