TCL stands for Tool Command Language and is pronounced "tickle". It is a scripting language that was created by Dr. John Ousterhout in UC Berkeley in the early 80's. From the begining, Ousterhout wanted an interpreted language that could be embedded into many different applications.
TCL is often accompanied by Tk, its graphical toolkit. Tk (and TCL with it) is remarkable for being widely cross platform, and being easy to program with. In fact, most major scripting languages like Perl, Python, and Ruby also have bindings to Tk.
Because of its simple syntax, TCL is a very easily extensible language. This has encouraged the development of many TCL extensions, including Expect, and incr TCL (an Object Oriented extension to vanilla TCL).
While TCL might have had its heyday in the early 90's, it remains a very useful programming language with a healthy community surrounding it. It is being actively developed primarily by a "core team" of 14 people. Dr. Ousterhout is still actively involved, and provides the final decision if the core team deadlocks.
The application domains that TCL is used is traditionally in GUI frontend applications and as the embedded language in scriptable applications. It is however also suited for jobs that most scripting languages are used for such as text processing, web scripting, and rapid prototyping.
TCL is released under a BSD-syle license.