A simple search in codex shows how to do that :
$args = array(
'post_type' => 'post',
'tax_query' => array(
array(
'taxonomy' => 'people',
'field' => 'slug',
'terms' => 'bob'
)
)
);
$query = new WP_Query( $args );
EDIT: do not use showposts
, it’s deprecated.