Create a folder inside sites/all named modules. This is where custom and contributed modules are usually housed.

The following list details the procedure involved in creating a module named mysite to hold our theme-agnostic customizations and other odds and ends:

Just as with themes, modules require a .info file which provides information to Drupal on compatibility, dependencies, and so on. Once Drupal ascertains that the module is compatible with the version installed, it loads the .module file of the same name and processes it accordingly.

We can test if the module is working by adding a snippet such as the following:

<?php
/**
* Implementation of hook_init().
*/
function mysite_init(){
// Display a message on every page load.
drupal_set_message("Welcome to MySite!");
}

As the comment suggests, the preceding snippet will display a welcome message on every page load.

The Drupal community routinely comes up with modules to ease the pain of development.