TCL stands for "Tool Command Language" and is a simple scripting language. Some argue that it is too simple...
Tk is a popular "GUI Library" designed for use with TCL; it is also usable with a variety of other languages, including C, Perl, Scheme, LISP, and Python, to mention only a few.
Travelocity web site uses TCL as one of the underlying programming languages.
Jim is an implementation of Tcl intended to have a very small footprint, only 14K lines of code. See also sources at msteveb/jimtcl @ GitHub
Using CTk, applications with a modern GUI-ish interface can be created for character terminals. These same applications, without modification, can provide a real GUI interface by using Tk. Thus, sites with an embedded base of character terminals (and a small capital budget) can smoothly migrate to GUI applications.
There are folks that don't much like TCL as a language; there was a pretty impressive Tcl War over this. Proponents of other scripting languages were pretty much agreed that their languages are superior to TCL, and trashed each other's languages over the design choices made in each case. Quite entertaining, if rather inconclusive.
Easier to agree on is that the Tk windowing "library" provides a useful set of operators that have proved to be useful in conjunction with a variety of languages.