Emacs started its life as “the extensible, customizable display
grew over the years into a full-blown ecosystem. Many tasks, usually
relegated to a diverse set of tools can be accomplished from within
Emacs in a consistent, familiar interface. Examples include
directory management, viewing PDF documents, editing files over SSH,
managing git repos,… (the list is quite long). In short, Emacs is
yours to make of it what you will: the spectrum of users varies from
those who use it to edit text files to extreme purists who use it to
virtually replace their operating system.
Emacs is extensible via a specialized dialect of Lisp known as Emacs
Lisp (Elisp) which has a lot of macros geared towards editing text
and managing text buffers. Any key (combination) you use in Emacs is
bound to an Emacs Lisp function and may be remapped to any other
function, including ones you write yourself.