IBM defined several communication protocols that are based on Telnet but modify and expand it. These protocols are used to connect to IBM Mainframe machines (TN3270 and TN3270E) and IBM AS/400 machines (TN5250 and TN5250E).
The difference between a Telnet session and a TN3270/TN5250 session is that a Telnet session uses the ASCII character set and sends a line of data at a time and a TN3270/TN5250 session uses the EBCDIC character set and sends a block of data (a screen refresh) at a time.
More information regarding these protocols can be found here: