In the world of Unix text editors, emacs is a special case. It is more than just an editor - it is a complete Lisp environment. It can do, with sufficient prodding, just about anything. Emacs has many sets of commands, each of which executes a particular function. Many of these functions are bound or attached to key sequences.
Key sequences are one or more keys, with or without a prefix key.
There are about eleven prefix keys in emacs, but typically only a few of these are used. The meta-key sequence, Esc, is also a prefix key. They are bound to the command self-insert, which inserts the key that is typed into the text buffer. There are a number of different variations of emacs. The one described in this handout is GNU Emacs.