Does TCL create software for Unix systems?
Is it like a programing language who gives a interface?
Can you tell me what it is used for.

TCL or Tool Command Language, is a general purpose programming language used on unix systems.

Tk, an extension to TCL, allows for the creation of graphical user interfaces.

In addition to what JTY said, Tcl is the base language for the well-known commercial Content Management System, Vignette (http://www.vignette.com). Because of the no. of companies + sites using Vignette, Tcl is useful in this respect.