Emacs is a text editor with a history. It was originally written by Richard Stallman, the guru of the free software GNU movement. Whatever you think of Stallman's personality, he's written some nice software. Emacs contains a powerful scripting language (Emacs Lisp) which has been used to write numerous extensions and plugins. At the moment it can do anything from running Python programs inside an editor window to reading newsgroups to playing games. It is also an excellent editor for editing source code ('modes' exist for almost every language, which add color highlighting, indentation, etc).
A big problem many people have with Emacs is that the best way (maybe even the only viable way) to use it is to know heaps of shortcut keys by heart. All of the functionality can be accessed by shortcut keys, but because there are only about 100 keys on your keyboard many functions require sequences of keys (for example CTRL-X CTRL-S saves your file). You'll have to be prepared for a period of reduced productivity while you are learning all these key combinations.
Still, once you are familiar with this editor it is a great way to code. Because everything goes with keys (mostly with the alphabetic keys you already have your hands on), you do not need to move your hands to the mouse or the arrows or wherever, which should reduce the pain in your wrists you get after those 8-hour coding sessions.
To get started, download the emacs binaries for your system from somewhere (look here for windows binaries), open the program and choose 'tutorial' from the help menu. This will show you the basics, the big online help system can help you with more complicated stuff.