Vim Directory Structure §

Directory	Description
autoload	for standard Vim autoload scripts
colors	colorschemes
compiler	contains Vim scripts for specific :compiler‘s. I.e options for errorformat.
ftplugin	Vim plugin scripts for specific filetypes.
indent	automatically compute the indent for a type of file.
keymap	Keymap files for vim
lang	language files for vim
macros	defined macros to test for vi compatibility
plugin	contains standard vim plugin scripts
syntax	syntax highlighting
tools	programs that work with vim, i.e vimspell
tutor	hands-on tutorial for vim
after	vim will load stuff in after after everything else

All files ending in .vim in this directory and subdirectories will be sourced by Vim when it detects the filetype that matches the name of the file or subdirectory.

Taken from http://www.panozzaj.com/blog/2011/09/09/vim-directory-structure/