You've seen several add_filter() and add_action() functions by now. These two functions allow you to take advantage of the hooks scattered throughout WordPress. For a complete list of available actions and filters, visit codex.wordpress.org/Plugin_API/Action_Reference and codex.wordpress.org/Plugin_API/Filter_Reference.
Filters allow you to modify or replace the output existing functions. For example, you could append ads or a copyright notice to content in feeds, or search and replace a word or phrase in your post/page content.
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