Who are Houthis, militia that defeated Saudi Arabia and took over Red Sea coast?

Iran-backed Houthis, who defeated Saudi-backed pro-government forces and seized Yemen’s entire Red Sea coast, are armed political/religious group that follows Zaydi sect of Shi'ite Islam. Founded in 1990s, the group is now led by Abdul Malik al-Houthi. They've taken control of Yemen’s entire coast along Bab el-Mandeb Strait, a critical lifeline for world's biggest oil exporter Saudi Arabia.