Hamas is the militant Sunni Islamic organization that controls the Gaza Strip.
The Islamic Resistance Movement, or Hamas, was founded in 1987, during the First Intifada, as a separation group from the Egyptian Muslim Brotherhood. The group is made up of Sunni Muslims and has controlled the Gaza Strip since 2006 when it won the Palestinian Parliament election, ousting Fatah. Hamas has a militant wing, known as the Izz ad-Din al-Quassan Brigades.
Hamas is indicated by the Secretary of State as a Foreign Terrorist Organization (FTO).
The United States is not alone. Canada, Japan, Israel, and the European Union all consider Hamas as some type of terrorist organization. Iran, Russia, Turkey, China, and an array of Arab countries have made no such allegations. Hamas is among al-Qaida, Boko Haram, and many other groups registered as a terrorist organization by the U.S. Department of State. You can see the full list here.