Difference between TCL and Shell?

The word "shell" can have two meanings here. First, remote access to the BIG-IP command line is usually accomplished via secure shell (SSH), which will dump you into a command shell. That command shell can either be the Linux Bash shell, or the Traffic Management shell (tmsh), depending on how your user account is configured. The TM shell is the command line interface to management operations, in parity with the management GUI and iControl methods

TCL, or "Tool Command Language" is the programming language that iRules are derived from.