Since 2000, there has been a five-fold increase in the number of people killed by terrorism. While there was a slight decline from 2007 onwards, the start of the civil war in Syria marked another jump in terrorist activity.
Terrorist networks can lie in wait for years before a major attack makes them a household name. In this shadowy world, many of the organisations are linked, and the most dangerous group may be the one we haven't heard of yet.
Here are 10 Islamist terror groups that have been in the news:
1. Islamic State in Syria and Iraq (ISIS)
Founded by Abu Bakr al-Baghdadi, Islamic State in Syria and Iraq (ISIS) holds large swathes of Iraq and Syria under its control. It is also known as the Islamic State in Syria and the Levant (ISIL), Islamic State (IS), or Daesh, which is seen as derogatory.
A few terrorist organisations have declared that their ultimate aim was to establish an Islamic "caliphate", but ISIS has made it a reality. They have committed widespread atrocities and instituted a brutal interpretation of Islamic law in areas they hold, which includes dozens of towns spread over Iraq and Syria.
According to reports, militants from Malaysia, Indonesia and the Philippines have formed a military unit in Syria which could expand their reach in South-east Asia.
The unit is called Katibah Nusantara Lid Daulah Islamiyyah, or Malay archipelago unit for the Islamic State in Iraq and Syria.
Al-Qaeda was formed in 1988 by Osama bin Laden, who was killed in 2011 in an operation by US Navy Seals. The group shot to infamy after the 2001 September 11 attacks, but it has been eclipsed in the past year by the Islamic State.
Since the death of Osama, the network has been led by Egyptian Ayman al-Zawahiri. While it appears to have lost its lustre recently, many of the groups in this list are affiliated to Al-Qaeda.
ISIS was itself part of the network, until it was formally ejected from Al-Qaeda early last year for being too brutal.
3. Al-Qaeda in the Arabian Peninsula (AQAP)
Formed in 2006 by the merger of the Yemeni and Saudi wings of al-Qaeda, al-Qaeda in the Arabian Peninsula (AQAP) is seen as one of the most dangerous offshoots of al-Qaeda.
Both brothers in the Charlie Hebdo attack in Paris on Jan 7 were trained by the group, also known as the al-Qaeda in Yemen.
It publishes an English-language propaganda magazine Inspire, which included Charlie Hebdo editor Stephane Charbonnier as a target in a hit list published in 2013.
While promoting attacks in the West, it has also managed to seize territory in Yemen, and trains fighters for extremist groups fighting in Syria and Iraq.
The Afghan Taleban was founded in 1994 under the leadership of Mullah Mohammed Omar, who remains its commander and spiritual leader. The organisation's main goal is to form an Islamic state in Afghanistan. It ruled Afghanistan from 1996 to 2001 during which it imposed strict syariah law.
It was overthrown during United States military action following the Sept 11, 2001 attacks. As the US withdraws its troops from Afghnistan, the Afghan Taleban has been making advances in the country again.
5. Pakistan Taleban
Last December, the Pakistani Taleban, also called Tehreek-e-Taleban Pakistan (TTP), stormed the Army Public School in Pakistan's north-western city of Peshawar on a weekday, slaughtering 148 people - including 132 children - in the country's deadliest terror attack.
They are also behind the shooting of Nobel Peace laureate Malala Yousafzai.
It is a loose coalition of militant groups operating from the semi-autonomous tribal zones in north-west Pakistan near the Afghanistan border. Its present leader is Maulana Fazlullah.
6. Al-Nusra Front
The Al-Nusra Front or The Front for the Defence of the Syrian People is sometimes known as al-Qaeda in Syria.
It announced its existence with a video posted online in 2012, and aims to replace the regime of President Bashar al-Assad with an Islamic state.
"We are Syrian mujahideen, back from various jihad fronts to restore God's rule on the Earth and avenge the Syrians' violated honour and spilled blood," a masked man declared in the video.
They have been actively involved in supporting Syrian rebels, and attacking targets affiliated with the Syrian government.
They are designated a terrorist organisation by Western nations, Saudi Arabia, Turkey and the UAE.
7. Boko Haram
Boko Haram aims to impose a hardline form of Islamic law in Nigeria.
The name means "Western education is sinful", and the group forbids Muslims from engaging in any activity related to Western society. This includes voting in elections, wearing shirts and trousers or receiving a secular education.
In January 2015, they began their deadliest massacre in Baga, a town in the north-east of Nigeria and as many as 2,000 have been killed, according to Amnesty International.
Last year, they kidnapped hundreds of students, including more than 200 schoolgirls who remain missing. The group has reportedly used women and young girls as human bombs in attacks.
Boko Haram controls about 20,000 square miles of territory in north-east Nigeria, The Telegraph reports in January.
8. Jemaah Islamiyah (JI) and splinter groups
Closer to home, the Jemaah Islamiyah (JI) is Al-Qaeda's branch in South-east Asia, and is responsible for the 2002 Bali bombing that killed 202 people.
It is based in Indonesia, and was formed in the early 1990s with the aim of establishing a caliphate in the region.
Many of its key figures, including spiritual head Abu Bakar Bashir, Bali bomb-maker Umar Patek and the leader of JI in Singapore, Mas Selamat Kastari have been captured.
The network has been severely degraded since 2002, but there were still a few attacks attributed to them last year.
The JI has now morphed into splinter groups such as the Jemaah Ansharut Tauhid (JAT). Last year, the Malaysian security authorities identified four new terror groups, known by their acronyms BKAW, BAJ, Dimzia and ADI. Most have pledged their allegiance to ISIS.
9. Abu Sayyaf
Abu Sayyaf, a criminal gang that operates in Sulu, is responsible for frequent kidnappings for ransom, along Sabah's coast and adjacent waters.
Set up in the 1990s with seed money from al-Qaeda, the group based in the islands of Basilan and Sulu, has been blamed for the worst terror attacks in the Philippines' history, including bombings and mass kidnappings of Christians and foreigners.
The group has long had links with al-Qaeda and recently pledged allegiance to ISIS.
Pakistani militant group Lashkar-e-Taiba (LeT) is responsible for the 2008 Mumbai attacks which killed 166 people, including one Singaporean.
Since the attack, it has used another organisation, the Jamaat-ud-Dawa (JuD) as a front organisation, reports say. The JuD claims it is a humanitarian charity, and continues to operate openly in Pakistan.
Pakistani journalist Arif Jamal takes another view.
He says that JuD is the central organisation, and it has sleeper cells not just in South Asia, but all around the world.
LeT is just one of its branches, which is responsible for waging jihad in India, he said in an interview with the Foreign Policy magazine last year.
Sources: Straits Times, New York Times, BBC, Global Terrorism Index, Foreign Policy