Conspiracy theories are adventure hooks that parties acquire as they move through the systems of the world, revealing the possibility that local power merchants have made agreements or arrangements by which they can further their control through channels outside the law.
Conspiracies are typically arranged between individuals who represent larger groups, such as the head of a guild, an officer of the law with minions or employees, an organization of like-minded persons and so on. Two or more such individuals will meet to work out a strategy in which their individual powers may be combined to ensure a better arrangement for all those involved. The groups represented by these individuals may know all, a part of or none of the actual conspiracy. Several persons in the group may be aware of only their end of the conspiracy; there may be many innocent persons in the group who are wholly unaware that illegal or immoral acts are being carried forth.
Very rarely, an organization may be designed for the sole purpose of conspiring with unwholesome or innocent persons alike - for the purpose of clarity, at a later time when I start writing about such organizations, I'll use the term cabal.
While the long-term goal of a conspiracy might be power, in the short-term conspiracies are typically founded with mundane expectations in mind: the removal of troublesome persons, the acquisition of territory or wealth, the establishment of security in one's position, the implementation of legislation, the instigation of war or participation in treasonous activities such as the selling of information or sabotage. While any of these may be accomplished within a few months, in the long term a successful conspiracy is likely to endure past its original mandate. Once a single action has been tried and managed, then other potential actions will spring to mind, encouraging participants to feel secure in their power, widen their conspiracy and attempt bolder, more risky operations.
Truly successful conspiracies may expand into purely evil endeavors - such as the spreading of lies to encourage mass fear or self-destructive behaviour. Individual lives can be destroyed through personal attacks and invented evidence. Persons may be outright assassinated, followed by invented justifications that encourage the people's passivity. Belief in the state can be undermined through the conspiracy's invention of a wholly false conspiracy designed to achieve desirable results. A policy of racism can lead towards participation in genocide. Anti-religious crusades based on the politics of threat and terror can initiate a warmongering social movement for the purpose of widespread conquest. The potential for conspiracies are endless.
Most often, conspiracies are founded by persons with few personal skills and some charismatic appeal who through ordinary channels have inserted themselves into a middle position within a heirarchy. Once there, they become dissatisfied with their lot. Very often the conspirator recognizes his or her personal limitati0ns and comes to understand that further success must be obtained by means unsavory. At this point they will cast about for an opportunity, generally finding it in another individual in some other branch of power suffering from the same limitations.
Able to work together, they balance their existing abilities against their willingness to swindle others, masquerade as persons of importance or otherwise evade expected practices. This puts them in a position to acquire followers who themselves see opportunities for improving their own existences. Very few of these followers will be more than thugs, but occasionally they will include an individual or two with personal ability in sharp dealing. Should an organization like this possess a reasonable organizational structure, strong bonds and the existence of a slight power vacuum - and some luck - it can be expected to expand considerably and endure for a time.
While the existence of a personal conspiracy may last only a lifetime, some conspiracies skip from generation to generation because they are founded upon a persistent idea that finds traction in new minds. Two typical examples would be persistent racism or the prophecy of future events that are unlikely to occur in anyone's specific lifetime. Either can be used as a jumping off point for making successful short-term gains possible.
An extensive list of conspiracy theories can be read on wikipedia.
Note that the newest changes I have made to my wiki (where the above is from) can be found here and also through a new addition to the links list.