See What is Tcl
Tcl Commands documents all the Tcl commands available in the core distribution.
Tcl C API documents all the public Tcl C calls that a C programmer should consider safe to use.
Tcl also comes with several packages. For instance, the http package, dde, msgcat, platform, registry, tcltest, the deprecated optparse package, as well as others [please fill them in here].
see Why Tcl
See this page for details about platforms known to support Tcl and Tk.
See Getting Tcl/Tk
See Installing tcl
See Documentation
See Learning Tcl
see Tcl Syntax
see Tcl Commands