When sending e-mails or newsletters from your website, you might want to inject some information like website name, disclaimer, copy right information, unsubscribe link and such. Just like hook_form_alter, you can use hook_mail_alter to virtually alter anything in the mail just before Drupal sends the e-mail. You can even change the mime type of the email, for example, from plain text to HTML and vice versa.
Here is the syntax/signature of the hook

• Drupal 5: hook_mail_alter(&$mailkey, &$to, &$subject, &$body, &$from, &$headers)
• Drupal 6, 7: hook_mail_alter(&$message)

$message has the following key-value pairs:

• 'id': Unique id of the mail being sent.
• 'to': The destination e-mail address(es)

The full view of a node including all the information associated with the nodes like author, node posted date, file attached to the node and any other data that are associated by other custom modules is managed by hook_view. The main goal and use of the hook_view is to allow the node modules to add extra information and manage those extra information to display when viewing the node. Nodes can have tease view as well as page view. On teaser view, only some of the information associated with the node is displayed; we can say its an overview of the node. On page view, all the information associated with the node is displayed.

<?php
/**
* Implementation of hook_view
* @param $node
* @param $teaser
* @param $page
* @return unknown_type
* @author Drupal Developer
*/

You've just published a Drupal website and people find it very useful. So, they started signing up with your website and its really very good traffic now. You have planned to implement some features and you want to let know about to all the members of the site. There are two ways: you send e-mails to individuals by using their contact form or implement a system with which you create a node and the site itself sends e-mails. How about sending the e-mails at specific period of time? ohh.. yeah. that's great. Drupal's hook_run makes it happen. Since, there are more features to come, you want to do this every time you have something new to let others know about, you would use hook_cron, which performs specific tasks assigned to it periodically.