Who are the janjaweed?

Much of the information in this answer comes from an essay by Brendan Koerner in slate.com. What he wrote in 2005 is still fairly accurate, although some of the violence has died down since then.

In the African country of Sudan, as well as in its neighboring country, Chad, there has been ongoing violence and civil strife, which has created more than 1 million refugees. The majority of that violence has been attributed to militias known as the Janjaweed. The word, an Arabic colloquialism, means "a man with a gun on a horse." (Some sources translate it as "a devil on horseback.") Janjaweed militiamen are primarily members of nomadic "Arab" tribes who've long been at odds with Darfur's settled "African" farmers, who are darker-skinned. Until 2003, the conflicts were mostly over Darfur's scarce water and land resources-desertification has been a serious problem, so grazing areas and wells are at a premium. In fact, the term "Janjaweed" has for years been synonymous with bandit, as these horse- or camel-borne fighters were known to swoop in on non-Arab farms to steal cattle.

The Janjaweed started to become much more aggressive in 2003, after two non-Arab groups, the Sudan Liberation Army and the Justice and Equality Movement, took up arms against the Sudanese government, alleging mistreatment by the Arab regime in Khartoum. In response to the uprising, the Janjaweed militias began pillaging towns and villages inhabited by members of the African tribes from which the rebel armies draw their strength-the Zaghawa, Masalit, and Fur tribes. (This conflict is entirely separate from the 22-year-old civil war that has pitted the Muslim government against Christian and animist rebels in the country's southern region. The Janjaweed, who inhabit western Sudan, have nothing to do with that war.)