Custom Taxonomy with tax_query

From the WordPress codex for wp_query:

Display posts tagged with bob, under ‘people’ custom taxonomy:

$args = array(
    'post_type' => 'post',
    'people' => 'bob'
);
$query = new WP_Query( $args );

Display posts tagged with bob, under ‘people’ custom taxonomy, using tax_query:

$args = array(
    'post_type' => 'post',
    'tax_query' => array(
        array(
            'taxonomy' => 'people',
            'field' => 'slug',
            'terms' => 'bob'
        )
    )
);
$query = new WP_Query( $args );

See the Codex page for more examples.