emacs is a text editor (as opposed to a word processor; it works with plain text, like the textboxes you type Hubski comments into), with a very long history, mostly used for editing programs. It can be customized by adding scripts in an (archaic) dialect of lisp. Modes are the most important sort of extension; they make emacs an environment for editing particular types of text, like different programming languages, HTML or TeX documents, todo lists, emails and blog posts. There are also modes that make it a web browser, irc client, game of tetris, terminal emulator, coffee pot controller ... The linked blog post is about an emacs extension for interacting with blogger, among other things. Rather than interacting with blogger through your browser, you do so through your text editor, making it convenient to keep local copies of your posts.