The process by which France began with a King and ended with an Emporer is a political phenonmena now known as a "Thermidorean Reaction." The ruler is deposed, but the revolution becomes so radical that eventually few support it, and the radical elements are overthrown in favor of reactionary ones. Most revolutions follow this pattern and end up worse from when they started. The Reign of Terror was overthrown in July, 1794. However, back then 1794 was considered as the second year within the French Revolutionary Calendar, and July/August were now a new month known as "Thermidor." This attempt to dechristianize the calendar did not last, and Napoleon reinstitued the more familar one shortly into his reign.
The previous post did a very solid job in addressing the question. I think that the interesting element in examining the French Revolution dynamic was the idea that a monarchy ended up being replaced with another monarchy. The forces that inspired the overthrowing of the French Royalty were powerful and very potent. Their intense nature led to a situation where establishing law and order became extremely difficult. The Reign of Terror and elements where individuals became disenchanted with the spirit of revolution caused them to embrace Napoleonic rule and, in a sense, compelled an abandoning of the anti- monarchic spirit that inspired the initial overthrow of the singular notion of leadership. In the most odd of ways, it seemed the more things changed, the more they stay the same.
The major cause of the French Revolution was the system of absolute monarchy that the French monarchy had been a part of.
Unlike the British, the French monarchy was absolute. There were no limits on it, no Parliament, none of the things that made the British monarchy way less oppressive. Because they were kept down so much, the commoners were very frustrated. This was especially true because of the spread of Enlightenment ideas that argued that all people should be free of tyranny.
In addition, the French monarchy spend lavishly on such things as palaces (and wars). Neither seemed to help the commoners. Yet the commoners were the ones who paid the taxes. So that was another major problem.