Emacs is more of an "environment" than just an editor, since it has the strong configuration language, emacs-lisp. There are a lot of applications written in emacs-lisp, and you may run many applications on your Emacs at the same time, i.e. e-mail reader, news reader, IRC client, a kind of IDE, etc. These applications likely consist of two or more windows, so when you switch among applications, you may want to save or restore how windows are located (this is called as "window-configuration"). For this purpose, Emacs has two functions, window-configuration-to-register and jump-to-register, but these are too primitive to use on a daily basis. ElScreen provides the ease-to-use environment to save or restore several window-configurations.