Recs.
Updated
The Emacs manual describes Emacs as "the extensible, customizable, self-documenting real-time display editor".
It includes a file manager as well as a number of useful utilities such as a function to find the difference between two files. An embedded shell can be used interactively or for calling external utilities in the editor environment (like grep, diff, gcc etc.)
SpecsUpdate
Pros
Pro Extremely customizable
Emacs is really an interpreter for its specialized Lisp dialect, Emacs Lisp. As such, it is very possible to manipulate the very fabric of the way things work in Emacs to your liking. The possibilities of what you can do with Emacs as compared to other software tools in general is quite staggering.
Pro Self documenting
Emacs has extensive help support built-in as well as a tutorial accessed with C-h t.
Pro Its age has led to many tools and an active community
Even though Emacs was created in the '70s, it is still in fairly wide use today and has a very broad range of tools and packages that are readily available for download and usage. The community is, even now, quite active and continues to create new tools that make the Emacs environment even more versatile and capable of just about anything.