The Ancien Regime system was based on the concentration of all political, social and economic power between three classes, the monarchy, the clergy, and the aristocracy. The First estate represented the Roman Catholic clergy. The Second estate represented the nobility, with its main governor, King Louis XVI and his "court". Finally, the Third estate represented the rest of the population, the poorest and largest group of people.
The inequality in distribution of powers and rights between these different estates was the main cause of the explosion of anger amongst Third estate representatives.
There were very few opportunities for the general population to ascend the social ladder as all privileges were determined by birth and not by talent. The social mobility rights could be bought but it was impossible for the poorest to afford them.
All the main political decisions were taken arbitrarily by the King and his advisers, with little or no direct consultation with the rest of the French population.
Added to this unfair and archaic political system, a terrible economic crisis pushed the poorest people closer to revolt.