See section Customizing Key Bindings . Whether a sequence is a key can be changed by customization. This manual is full of passages that tell you what particular keys do. But Emacs does not assign meanings to keys directly. Instead, Emacs assigns meanings to named commands , and then gives keys their meanings by binding them to commands.
Every command has a name chosen by a programmer.
For more information on commands and functions, see section 'What Is a Function' in The Emacs Lisp Reference Manual . The bindings between keys and commands are recorded in various tables called keymaps . See section Keymaps . It is the command next-line that is programmed to move down vertically. C-n has this effect because it is bound to that command.