A cabal is a group of people which meets for the purpose of plotting something which is typically illegal or simply evil. Cabals have obviously been a part of human society for thousands of years, working behind the scenes to perform various political and social intrigues, ranging from assassinations to insider trading. As a general rule, when a cabal is outed, the results are catastrophic, as members struggle to conceal their involvement and the goals of their organization.
The term was first used in reference to a pack of scheming ministers in the administration of British King Charles II. The ministers worked from around 1671-1673, and their collective actions became so associated with conspiracy that they loaned the first letters of their last names to the acronym C.A.B.A.L, which later became “cabal.” Oddly enough, the members of this original cabal actually didn't get along very well, perhaps because they were too busy advancing their own schemes through treachery and intrigue. The word is also said to be linked with Kabbalah, a mystical Jewish tradition which is shrouded in history.
Lord Clifford, the “C” in cabal, was a social climber who fought bitterly to reach his position as Lord Treasurer. He converted to Catholicism, and turned out to be a bit of a war monger, suggesting invasion of the Netherlands to stock the national treasury, and presumably his own coffers as well. Lord Arlingon became famous for pursuing ultimate power for Charles II, and also agitating for war with the Dutch, while Lord Buckingham plotted to become chief minister, ruthlessly undermining those in his way. Lord Ashley intrigued against the accession of James II, the next king, while Lord Lauderdale was a ferocious and cynical Scottish governor.
When a cabal becomes powerful enough, it may go public, taking advantage of popular opinion and a large member base to advance itself. In some cases, cabals are actually quite open about their goals, especially in politics, although members of the group may not tell the whole story behind their actions and motivations. Even when a cabal goes public, the implication is that back room dealing is involved, adding a certain shadowy aspect to the activities of the group.
The term is also used in the technology world to describe shadow groups which may control message boards and games; without a cabal, many of these media would sink into anarchy, so the implication of insidious plotting is perhaps undeserved. A cabal can also be a group of people who work closely together on product development in isolated environments, in the slang terminology of some industries.