I was trying to install autoclose.vim to Vim. I noticed I didn't have a ~/.vim/plugin folder, so I accidentally made a ~/.vim/plugins folder (notice the extra 's' in plugins). I then added au FileType python set rtp += ~/.vim/plugins to my .vimrc, because from what I've read, that will allow me to automatically source the scripts in that folder.  
The plugin didn't load for me until I realized my mistake and took out the extra 's' from 'plugins'. I'm confused because this new path isn't even defined in my runtime path. I'm basically wondering why the plugin loaded when I had it in ~/.vim/plugin but not in ~/.vim/plugins?
All folders in the rtp (runtimepath) option need to have the same folder structure as your $VIMRUNTIME ($VIMRUNTIME is usually /usr/share/vim/vim{version}). So it should have the same subdirectory names e.g. autoload, doc, plugin (whichever you need, but having the same names is key). The plugins should be in their corresponding subdirectory.
Let's say you have /path/to/dir (in your case it's ~/.vim) is in your rtp, vim will

look for global plugins in /path/to/dir/plugin
look for file-type plugins in /path/to/dir/ftplugin
look for syntax files in /path/to/dir/syntax
look for help files in /path/to/dir/doc

and so on...
vim only looks for a couple of recognized subdirectories† in /path/to/dir. If you have some unrecognized subdirectory name in there (like /path/to/dir/plugins), vim won't see it.
† "recognized" here means that a subdirectory of the same name can be found in /usr/share/vim/vim{version} or wherever you have vim installed.
Vim plugins don't always load?