Together with actions, filters make up the two different types of hooks in WordPress. They allow plugins and themes to modify or replace the contents of a variable created by the WordPress core, or by other plugin/theme.

For example, if you wanted to add a signature to the end of every post, you could use the the_content filter like this:

function addSignatureToPosts( $content )
    return $content . '<p>Your custom signature goes here.</p>';
add_filter( 'the_content', 'addSignatureToPosts' );
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