The Sulu Islands are a region of the Philippines, an island chain that stretches from near Zamboanga, at the southwestern tip of Mindanao, almost to the Malaysian province of Sabah, at the eastern end of Borneo. Politically, they are part of the Philippines but culturally they are close to Malaysia and Indonesia; in particular they are almost entirely Muslim.
WARNING: This is probably the most dangerous area in the Philippines; all western governments advise strongly against travel to this region. Back in the 19th century, Sulu was a pirate kingdom and Jolo had a huge slave market. Today, there are still some pirates and other bandits. Perhaps more dangerous, there are also several armed Moro (Filipino Muslim) independence groups, including some radical Islamists. See the warning at the top of the ARMM page for more information.