Some people might say there is a holy war going on between vi and emacs fans. The author of this document is not religious, she just happened to have learned vi first and stuck to it, and wishes to make clear that emacs is as good as vi or vice versa.
Emacs is a very powerful editor, it allows you to even to use it as a client for mail and news and offers any number of tools to make life more comfortable. There are versions for almost any platform available (also non-Unix).
See the booklist in Appendix 2 if you want more information on the emacs editor.