Tk refers to Toolkit and it provides cross platform GUI widgets, which helps you in building a Graphical User Interface. It was developed as an extension to Tcl scripting language by John Ousterhout. Tk remained in development independently from Tcl with version being different to each other, before, it was made in sync with Tcl in v8.0.
It is cross platform with support for Linux, Mac OS, Unix, and Microsoft Windows operating systems.
Large successful applications have been built in Tcl/Tk.