tcl means tool command language and tk is the graphical user interface for it and other languages.
There is shell that can be started in a console tclsh (or under Windows tclsh86.exe). The command exit brings you back. Alternatively you can write everything in a file
puts "Hello World"
and tclsh hello.tcl or add the magic line pointing to the interpreter
#! /usr/bin/tclsh puts "Hello World"
Set run permission on this file and run it as ./hello.tcl
Tk is not limited to just tcl it can also be used by other programming languages as python, where
import tkinter