The autoload directory is for standard Vim autoload scripts.

These are functions used by plugins and for general use. They will be loaded
automatically when the function is invoked. See ":help autoload".

gzip.vim for editing compressed files
netrw*.vim browsing (remote) directories and editing remote files
tar.vim browsing tar files
zip.vim browsing zip files
paste.vim common code for mswin.vim, menu.vim and macmap.vim
spellfile.vim downloading of a missing spell file

Omni completion files:
ccomplete.vim C
csscomplete.vim HTML / CSS
htmlcomplete.vim HTML
javascriptcomplete.vim Javascript
phpcomplete.vim PHP
pythoncomplete.vim Python
rubycomplete.vim Ruby
syntaxcomplete.vim from syntax highlighting
xmlcomplete.vim XML (uses files in the xml directory)