#theme

The #theme property ties in strongly with what we've been talking about earlier in this chapter—theme hooks. It specifies that the render array needs to render some kind of data using one of the theme hooks defined. Together with this property, you will usually encounter other properties that map to the name of the variables the theme hook has registered in hook_theme(). These are the variables the theme system uses to render the template.

This is the property you will use in your business logic to convey that your data needs to be rendered using a specific theme hook. If you thought that you can only use theme hooks you registered, you'd be incorrect. There are many theme hooks that have been already registered by Drupal core and also contributed modules that make the life of a Drupal developer much easier. Just look inside drupal_common_theme() for a bunch of common theme hooks that you can perhaps use.