More than one million people have fled Sudan's Darfur region, the victims of what UN officials have described as an "ethnic cleansing" campaign by a group of Arab militiamen.
The Janjaweed often fight on horseback, or use camels
Many thousands have been killed and human rights groups say there has been a systematic campaign of rape, intended to humiliate and punish non-Arab groups.
The Janjaweed have attacked black Africans from the Fur, Massaleet and Zagawa ethnic groups with a ruthlessness that has not been seen in the region for some time, report aid agencies and the refugees themselves.
They have killed, raped, maimed, looted and burned down tens of thousands of village homes, displacing hundreds of thousands of people.
The BBC's Alfred Taban in Khartoum says the number of the Janjaweed is reported to be very small, maybe a few thousand, but they are well armed with automatic weapons and ride well-fed horses and camels.
Correspondents say that many of the men fighting with the Janjaweed received military training in Libya in the 1980s, when Muammar Gaddafi set up an "Islamic Legion" of mercenaries recruited from across north and west Africa.
Even before the rebellion erupted in Darfur more than two years ago, the Janjaweed, known then as Arab tribesmen, had been raiding African villages, our correspondent says.
The objective then, as it is probably still now, is to drive the African tribesmen from their homes and force them to abandon valuable water points and pasture.
The Janjaweed are nomads and they have been hard hit by desertification, which has greatly diminished water resources and pasture in Darfur.
Human rights campaigners say the Janjaweed militia were armed and recruited by the Sudan government.
The government denies these claims but says that self-defence groups were formed following attacks by rebels groups, who took up arms to demand more rights for Darfur's non-Arab groups.
The United States is threatening to impose sanctions on seven, named alleged Janjaweed leaders.
One of them, Musa Hilal, vehemently denies taking part in ethnic cleansing.
"The rebels spread the word Janjaweed as if it were an organisation. As a political group there is no specific concept called Janjaweed... It means nothing, but has been used to mean everything," he told the UK's Guardian newspaper.
He said that he had responded to a government call to fight back against Darfur's rebels.
"The government was putting forward a programme of arming for all the people. I called our sons and told them to become armed," he said.
Following intense international pressure, the government promised to disarm the Janjaweed.
But, our correspondent says whether that will indeed be done is another question.
The African tribesmen have supplied the bulk of the fighting forces for both the Sudan Liberation Army (SLA) and the Justice and Equality Movement (Jem), the main rebel groups in Darfur.
Both refugees and the Janjaweed use camels to get around
By scuppering the African tribesmen, the Janjaweed are able to disrupt the two movements' recruitment drives.
This fits well in the government strategy of beating the rebels militarily.
The Africans of Darfur and relief agencies say that, far from stopping the Janjaweed, the government is providing them with weapons, training and uniforms.
But the first group of Janjaweed has been found guilty of murder and armed robbery.
Ten men have been sentenced to have their left hands and right feet amputated, and to spend six years in prison.
The government says this is proof that it is, finally, taking action to rein in the militia.