Others have already described TeX quite well. One important feature not mentioned is that it is virtually error free now and is frozen at version 3.14159... (ye

Others have already described TeX quite well. One important feature not mentioned is that it is virtually error free now and is frozen at version 3.14159... (yes pi) Donald Knuth decided that there is value in having a system that for all time will produce exactly the output the author intended 20 years or 200 years later. Every TeX system will layout a document in exactly the same way within the limits of the output device—internally TeX computes positions with fixed point arithmetic at units smaller than light wavelengths. There is a tremendous ecosystem that runs on top of TeX written in the TeX macro based language. This includes the popular LaTeX system, hundreds of packages including tools for example to typeset musical scores, chess articles, and everything else you can imagine. Do note that some of these packages are very high quality—many comercial books in difficult to typeset subject like Physics, Mathematics, and CS are written using TeX.