Tcl (Tool Command Language) is a very powerful but easy to learn dynamic programming language, suitable for a very wide range of uses, including web and desktop applications, networking, administration, testing and many more. Open source and business-friendly, Tcl is a mature yet evolving language that is truly cross platform, easily deployed and highly extensible.
Tk is a graphical user interface toolkit that takes developing desktop applications to a higher level than conventional approaches. Tk is the standard GUI not only for Tcl, but for many other dynamic languages, and can produce rich, native applications that run unchanged across Windows, Mac OS X, Linux and more.
InstallJammer is written almost entirely in Tcl/Tk with a little bit of help from some extensions written in C. Since all scripting in InstallJammer is done through Tcl/Tk, it might be helpful for you to learn a little about Tcl if you plan on doing any kind of complex operations with your installer. Below are some links to get you started.