Drupal 7 Module Development

Chapter 3
After the data has been retrieved and manipulated in the heart of your module (the
business logic), it will need to provide the default theme implementation. Sometimes
a particular theme will need to override your implementation in order for it to
achieve a specific design goal; if the theme provides its own implementation, Drupal
will use the theme implementation instead of the module's default implementation.
When building our first module in Chapter 2, we saw a brief example of this in action
as follows:
$variables = array('items' => $list, 'type' => 'ol');
$content = theme('item_list', $variables);
By calling the theme() function, we are delegating the responsibility of determining
and using the proper theme implementation. We're saying:
"Hey, theme()! I want to markup my data as an item_list. Can you do that for me?
I don't need to know the details. kthxbye."
Our module just needs to decide which theme hook it wants to use to markup its
data. Should the data be displayed in an unordered list, a table, or a wordle?
Hook crazy?
In addition to API hooks, Drupal also has theme hooks. A theme
hook is simply the name of a particular way to markup some data. For
example, passing data to the item_list theme hook will result in
different markup then passing data to the links theme hook. However,
while normally every module's hook function will be called when Drupal
invokes an API hook, only one theme hook implementation will be
invoked when Drupal invokes a theme hook.
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