WordPress allows developers to add their own code during the execution of a request by providing various hooks. These hooks come in the form of actions and filters:

Actions: WordPress invokes actions at certain points during the execution request and when certain events occur.

Filters: WordPress uses filters to modify text before adding it to the database and before displaying it on-screen.

action-filters

The number of actions and filters is quite large, so we can’t get into them all here, but let’s at least take a look at how they are used.

Here’s an example of how to use the admin_print_styles action, which allows you to add your own stylesheets to the WordPress admin pages:

And here’s how you would use the the_content filter to add a “Follow me on Twitter!” link to the bottom of every post: