Hamas, which will form the next Palestinian Authority government, has an ideology that is based on the destruction of Israel through jihad, or Muslim "holy war." The group's 1988 charter states that "Israel will exist and will continue to exist until Islam will obliterate it, just as it obliterated others before it."
It adds that the territory of Israel is "Islamic Wakf" -- part of the Muslim religious trust that cannot be given to non-Muslims -- and that "the law governing the land of Palestine is the Islamic Sharia," or Muslim law.
The group presents itself as having separate social and military branches, a formula that seeks to insulate the group from charges that it is a terrorist organization. However, few serious observers believe the branches are truly separate.
Hamas has its origins in the Muslim Brotherhood, a fundamentalist Muslim group founded in Egypt in the first half of the 20th century. The brotherhood inspired Hamas founder Sheik Ahmed Yassin's notion that Israel is Islamic land whose ownership is not negotiable.
Yassin founded the Islamic Center in the Gaza Strip in the 1970s, turning it into a major religious organization and laying the groundwork for a network of social and welfare institutions that increased the movement's popularity.
He continued to absorb the violent and nationalist ideas of the Muslim Brotherhood and gradually shifted the group's focus from welfare to violence. That paved the way for the founding of Hamas -- which means "zeal" in Arabic and is an acronym for the Islamic Resistance Movement -- after the first intifada began in 1987.
As early as the first intifada, Hamas also targeted suspected Palestinian collaborators and rivals in the Fatah movement.
Hamas began using suicide bombers as a weapon in 1994 and since has carried out at least 60 such attacks. Many more have been stopped by Israeli security forces. The group began launching rockets at Israeli targets in 2001, using crude Kassam rockets to shell Israeli towns in the Negev, notably Sderot.
The group's attacks have killed hundreds of Israeli civilians in the past five years alone, prompting Israeli legal and military responses. The United States and European Union consider Hamas a terrorist organization.
An Israeli court sentenced Yassin in 1984 to 13 years in jail, but he was released a year later in a prisoner exchange deal. He was imprisoned again in the 1990s for incitement to violence but was released in 1997 in another prisoner exchange.
During the second intifada, the Israel Defense Forces began targeting Hamas leaders for assassination. Yassin was killed in March 2004 by Israeli helicopter fire. Abdel Aziz Rantissi, who was appointed Hamas head in Yassin's place, was assassinated a month later.
After that, Hamas stopped announcing the names of its leaders, though they are believed to be Mahmoud al-Zahar and Ismail Haniya, No. 1 and No. 2 on Hamas' party list in the recent election.
The group's popularity in the territories is partly based on its social service work. Hamas funds educational, medical and welfare programs, though the group is accused of using the educational program to spread anti-Israel and extremist Islamic propaganda to children.
Hamas attempted to take credit for Israel's 2005 withdrawal from Gaza and parts of the West Bank.
The group has a few senior leaders in Syria, Lebanon and the Persian Gulf states. Hamas receives some funding from Iran but relies primarily on donations from Palestinians around the world and private benefactors in Arab states.
Some of Hamas' fundraising and propaganda activity takes place in Western Europe and North America. In 2004, the United States convicted the Texas Holy Land Foundation on charges that included money laundering for Hamas.
Israeli intelligence in the past has pointed at possible links between Hamas and Al Qaeda and Hezbollah, but nothing has been proven.