The “plugin” directory is for plugins that should always be loaded whenever Vim starts. These define global commands, interface tweaks, or new functionality useful for all or many file types.

The “ftplugin” directory is for filetype-specific plugins, such as autocompletion or highlight-errors-as-you-edit plugins.

The “syntax” directory is for updated syntax highlighting rules specific to a file type.

The “indent” directory is for filetype-specific indentation rules.

The “ftdetect” directory is for filetype-detection rules. Files that go here should usually do nothing but set the filetype so that files in the ftplugin, indent, or syntax directories are loaded for a new filetype.

The “colors” directory is for color scheme files.

The “doc” directory is for any help file documentation.

The “after” directory contains another tree with directories like “plugin”, “ftplugin”, and “syntax” that can be used to override settings in the system-wide settings or other plugins placed in the normal version of these directories. These files will always be loaded after their counterparts in other directories.

All about the VIM Folder structure