The 'tax_query' clauses can be nested, to create more complex queries. Example: Display posts that are in the quotes category OR both have the quote post format AND are in the wisdom category:

$args = array(
	'post_type' => 'post',
	'tax_query' => array(
		'relation' => 'OR',
		array(
			'taxonomy' => 'category',
			'field'    => 'slug',
			'terms'    => array( 'quotes' ),
		),
		array(
                        'relation' => 'AND',
                        array(
			        'taxonomy' => 'post_format',
			        'field'    => 'slug',
			        'terms'    => array( 'post-format-quote' ),
                        ),
                        array(
                                'taxonomy' => 'category',
                                'field'    => 'slug',
                                'terms'    => array( 'wisdom' ),
                        ),
		),
	),
);
$query = new WP_Query( $args );

Display posts from several custom taxonomies:

$args = array(
	'post_type' => 'post',
	'tax_query' => array(
		'relation' => 'AND',
		array(
			'taxonomy' => 'movie_genre',
			'field'    => 'slug',
			'terms'    => array( 'action', 'comedy' ),
		),
		array(
			'taxonomy' => 'actor',
			'field'    => 'term_id',
			'terms'    => array( 103, 115, 206 ),
			'operator' => 'NOT IN',
		),
	),
);
$query = new WP_Query( $args );

Simple Taxonomy Query:

Display posts tagged with bob, under people custom taxonomy:

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