Key-value pairs containing data for posts. Can be managed by plugins or sometimes directly by administrators.
The same, but for users instead of posts.
(e.g. upon initialization of the plugin)
Themes must contain an index.php and a style.css
Generally they should be used to provide style and not functionality, but any PHP and hooks are available for developers to use.
Plugins have access to the same action hooks as themes, and typically contain functionality changes which might include default styles.
Plugins can also run code upon activation/deactivation
*be careful migrating your data - remember that serialized data store in post meta?