Emacs is an open source text editor which is used mainly by luddite computer programmers. It is designed to be arcane enough that it requires an extensive amount of time to learn to operate efficiently.
Emacs Lisp
Emacs Lisp (often known to professionals as elisp) is the programming through which Emacs operates. The majority of Emacs features and tools are written in elisp, running above an Emacs kernel that is written in C. Emacs users commonly spend a great deal of time customizing the elisp portion of Emacs in order to better tailor Emacs to their needs.