I am having trouble understanding this SQL statement. Can someone shed some light on the subject for me? I wish to modify the statement to extract more data than it does... What i would like to do is either simplify/reformat the statement or understand what all the u.user and ". USERS_TABLE stuff is all about... please help me

Code:
SELECT t.*, u.username, u.user_id, u2.username as user2, u2.user_id as id2, p.post_username, p2.post_username AS post_username2, p2.post_time
        FROM " . TOPICS_TABLE . " t, " . USERS_TABLE . " u, " . POSTS_TABLE . " p, " . POSTS_TABLE . " p2, " . USERS_TABLE . " u2
        LEFT JOIN " . FORUMS_TABLE . " f ON t.forum_id=f.forum_id 
        WHERE t.topic_poster = u.user_id AND p.post_id = t.topic_first_post_id AND p2.post_id = t.topic_last_post_id AND u2.user_id = p2.poster_id
        ORDER BY t.topic_last_post_id DESC 
        LIMIT $limit