|Boko Haram threatening to sell the kidnapped girls.|
Its followers are said to be influenced by the Koranic phrase which says: "Anyone who is not governed by what Allah has revealed is among the transgressors".
Boko Haram promotes a version of Islam which makes it "haram", or forbidden, for Muslims to take part in any political or social activity associated with Western society.
This includes voting in elections, wearing shirts and trousers or receiving a secular education. Boko Haram regards the Nigerian state as being run by non-believers, even when the country had a Muslim president.
But residents in the north-eastern city of Maiduguri, where the group had its headquarters, dubbed it Boko Haram. Loosely translated from the local Hausa language, this means "Western education is forbidden".
They still refuse to send their children to government-run "Western schools", a problem compounded by the ruling elite which does not see education as a priority.
Against this background, the charismatic Muslim cleric, Mohammed Yusuf, formed Boko Haram in Maiduguri in 2002. He set up a religious complex, which included a mosque and an Islamic school.
Many poor Muslim families from across Nigeria, as well as neighbouring countries, enrolled their children at the school. But Boko Haram was not only interested in education. Its political goal was to create an Islamic state, and the school became a recruiting ground for jihadis.
In 2009, Boko Haram carried out a spate of attacks on police stations and other government buildings in Maiduguri. This led to shoot-outs on Maiduguri's streets. Hundreds of Boko Haram supporters were killed and thousands of residents fled the city.
Nigeria's security forces eventually seized the group's headquarters, capturing its fighters and killing Mr Yusuf. His body was shown on state television and the security forces declared Boko Haram finished.
Boko Haram's trademark was originally the use of gunmen on motorbikes, killing police, politicians and anyone who criticises it, including clerics from other Muslim traditions and Christian preachers.
The group has also staged more audacious attacks in northern and central Nigeria, including bombing churches, bus ranks, bars, military barracks and even the police and UN headquarters in the capital, Abuja.
Amid growing concern about the escalating violence, President Goodluck Jonathan declared a state of emergency in May 2013 in the three northern states where Boko Haram is the strongest - Borno, Yobe and Adamawa.
The deployment of troops has driven many of the militants out of Maiduguri, their main urban base and they have now retreated to the vast Sambisa forest, along the border with Cameroon.
From there, the group's fighters have launched mass attacks on villages, looting, killing and burning properties in what appeared to be a warning to rural people not to collaborate with the security forces, as residents of Maiduguri had done.
Boko Haram has also stepped up its campaign against Western education, which it believes corrupts the moral values of Muslims, especially girls, by attacking two boarding schools - in Yobe in March and in Chibok in April.
It abducted more than 200 schoolgirls during the Chibok raid, saying it would treat them as slaves and marry them off - a reference to an ancient Islamic belief that women captured in conflict are part of the "war booty".
|The remains of Chibok School after Boko Haram attack.|
At the same time, Boko Haram has continued with its urban bombing campaign, targeting the capital on 14 April, when at least 70 people were killed in an explosion near a car park and on 2 May when 19 people died.
This shows that not only does Boko Haram have a fighting force, but also cells that specialise in bombings. Analysts say northern Nigeria has a history of spawning militant Islamist groups, but Boko Haram has outlived them and has proved to be far more lethal, with a global jihadi agenda.
The threat will disappear only if Nigeria's government manages to reduce the region's chronic poverty and builds an education system which gains the support of local Muslims, the analysts say.
Nigeria: A nation divided