From the home page of csound (http://www.csounds.com ):
Csound is a sound design, music synthesis and signal processing system, providing facilities for composition and performance over a wide range of platforms. It is not restricted to any style of music, having been used for many years in the creation of classical, pop, techno, ambient, experimental, and (of course) computer music, as well as music for film and television.
And the handbook (http://www.csounds.com/manual/html/index.html , chapter TclCsound) continues:
TclCsound was introduced to provide a simple scripting interface to Csound. Tcl is a simple language that is easy to extend and provide nice facilities such as easy file access and TCP networking. With its Tk component, it can also handle a graphic and event interface. TclCsound provides three ‘points of contact' with Tcl:
1. a csound-aware tcl interpreter (cstclsh)
2. a csound-aware windowing shell (cswish)
3. a csound commands module for Tcl/Tk (tclcsound dynamic lib)
Cecilia is a program to make sound. Sound in its widest definition. Sound as music, sound as composition, sound as accessory.
Cecilia is a general sound-processing environment Cecilia is a graphical interface Cecilia is a text editor Cecilia is a music programming language
Cecilia serves the composer with little programming experience and the programmer with little composing experience. There is very little, sound wise, that cannot be done with Cecilia. Cecilia operates on top of Csound, the audio/dsp engine from Barry Vercoe at MIT.
and
All of Cecilia is constructed in tcl/tk, a slow yet appropriate scripting language as Cecilia does no audio computations.