Example of filtering by multiple categories and tags with a custom WP Query

gistfile1.php

<?php
$categories = $_GET( 'categories' );
$tags = $_GET( 'tags' );

$query = new WP_Query(
    array( 'cat' => $categories ),
    array( 'tags' => $tags )
);

while ( $query->have_posts() ) :
    $query->the_post();
    echo '<li>' . get_the_title() . '</li>';
endwhile;
?>

<a href="/?tags=cake,cookie,baking&categories=14,92,16">Link to custom query</a>