Emacs is an open source text editor which is used mainly by computer programmers. It is designed to provide a suite of tools which often require extensive programming knowledge to understand or exploit fully.
Emacs Lisp
Emacs Lisp (often known to professionals as elisp) is the prominent 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. Professional Emacs users commonly spend a great deal of time customizing and supplementing the elisp portion of Emacs in order to better tailor Emacs to their needs.