If you want to only display posts from a certain category id (or array of category ids), it’s just one extra line in the WP_Query arguments, like this:
$query = new WP_Query( 'cat=2,6,17,38' );
If you don’t want to use ID’s, you can do this:
$query = new WP_Query('category_name=staff');
You can of course combine this with the sorting by date from above – this is just one of the arguments.
