Who are Yemen's Houthis and what do they want?

Who are Yemen's Houthis and what do they want? Officially known as Ansar Allah, or "helpers of God", the movement emerged in Yemen's far north in the 1990s around the cleric Hussein al-Houthi. It is rooted in Zaydism, a branch of Shia Islam whose followers are a minority in Yemen but concentrated in the northern highlands.