Hamas stands for Islamic Resistance Movement and in Arabic means "zeal".
It was founded in 1987 by Shaikh Ahmad Yassin and Abdel Aziz al-Rantissi as an offshoot of the then Muslim Brotherhood organisation during the first intifada against Israel.
It is formally committed to establishing a Palestinian state in its own borders and to the destruction of Israel. It has pursued this aim through a series of attacks on Israeli troops, settlers and civilians both in the Palestinian territories and in Israel.
Hamas first came to prominence due to its opposition of the Oslo peace accords in 1994. It is not clear how many members are there in its political and military wings but undoubtedly it has many supporters.
It did not take part in the first parliamentary elections in 1996 due to its opposition to the Oslo accords.
The US regards Hamas as a
In the same year the group claimed responsibility for a series of bombings that killed 50 Israeli civilians and derailed the peace process.
Both Yassin and al-Rantissi were killed by separate Israeli military strikes in 2004, part of a clampdown on perceived terror groups.
Khaled Meshaal is now said to be in overall control and is based in Syria
However since the death of Yasser Arafat, Hamas has taken part in local and municipal elections with some success.
Besides its militant wing, Hamas also runs a series of charities that focus on social aid and welfare programmes.
Since the second intifada that began in 2000, Hamas has sought to give aid and schooling to victims in settlements and refugee camps, something the Palestinian Authorirty has often been unable to do.
Unlike the Fatah, Hamas is seen to be corruption free and not riven with disagreements, something that must have appealed to Palestinian voters.
Since early last year Hamas has gone further than other militant organisations in respecting a ceasefire that Palestinian President Mahmoud Abbas brokered with Israel.
Its electoral success now puts into doubt US-backed peace efforts.
Washington brands Hamas a terrorist organisation and has steadfastly refused to do business with it.
It is unclear what Israel's and the US positions will now be.
Head of the Hamas electoral list. He was born in Gaza's Shati refugee camp and graduated from Gaza City's Islamic University.
Haniya says Hamas has won
more than 70 seats
He became a close associate of Hamas founder Shaikh Ahmad Yassin before being expelled to south Lebanon in 1992. He returned to Gaza a year later and became the dean of the Islamic University.
In 1998, he took charge of Yassin's office and served as a liaison between Hamas and Palestinian Authority, established in 1994.
He rose to prominence after Israel's assassinations in 2004 of Yassin and his successor, al-Rantissi.
He has been a member of the political leadership of Hamas since the 1990s.
Mohammed Abu Tier
Second on the Hamas list, the 55 year-old spent 25 years in Israeli jails. He is from Jerusalem, and is a former member of Fatah and of the radical Popular Front for the Liberation of Palestine-General Command.
He sports a bright orange beard, dyed with henna in line with Islamic tradition, and preaches in local mosques in the neighbourhood of Um Tuba on the edge of Jerusalem.