A scripting language embedded in many applications, much like Elisp is used in Emacs and VisualBasic is in many MicrosoftWindows applications. It is not well suited for writing large programs, but because it is almost always combined with the Tk ToolKit, it's a good choice for writing portable GUI-based scripts. Most people would probably consider it obsolete; the dwindling popularity of Tk only contributes to this.
Perl, Python and Ruby are better choices for complex codebases, while GTK and Qt nowadays lead the ToolKit pack, though no combination of these makes GUI-based scripts as easy to deploy as TclTk.
See also:
Tcl Tutorial
The Tcler's wiki, a useful resource on Tcl.
Manual Pages
Tcl Developer Xchange
The Tcl newsgroup
Tclwise, a partially online book about Tcl, may be a little challenging to newcomers but worthwhile.CategoryProgrammingLanguages, CategoryVeryHighLevelProgrammingLanguages
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