Theorema was conceived and initiated around 1995 by Bruno Buchberger and reflects his view of "doing mathematics". It is being developed under his guidance by the Theorema Working Group at the Research Institute for Symbolic Computation, Johannes Kepler University, Linz - Hagenberg, Austria. Theorema 2.0 is a major re-launch mainly developed by Wolfgang Windsteiger.
Theorema is a Mathematica package implemented in the Mathematica programming language. The repository contains an entire Wolfram Workbench project for system development, i.e. if you clone the repository into a fresh directory, then the existing project can be imported into the Workbench's workspace from that directory. The project contains a directory "Theorema", which is the base directory of the Mathematica package itself making up the Theorema system.
This repository serves for the development of the Theorema system, it is the thing you need if you want to contribute to system development. If you only want to use the system, you are probably better off if you download just the Theorema package from the Theorema website. For more information, please visit http://www.risc.jku.at/research/theorema/software/.
http://www.risc.jku.at/research/theorema/software/documentation/tutorial/ProgrammersGuidelines.html