Parlements were political institutions that developed of the previous "Kings Councils, the Conseil du Roi or Curia Regis. Originally there was just one Parlement, that in Paris, but by mid Fifteenth Century there was one in Toulouse, which extended its authority over much of Southern France. From 1443 until the explosion of the French Revolution there were fourteen other parlements created, in cities such as Arras, Grenoble and Perpignan. Importantly, all these cities had always been administrative capitals of their regions (often stemming from Roman rule) and had strong traditions of independence from central control.
Officially parlements were not legislative bodies, but rather courts of appeal. However, they did have the responsibility to record all edicts and laws, and could refuse to apply such laws when they went against "fundamental law", or the local coutumes. Increasingly, and this was particularly the case with the Parlement of Paris, the parlements began to "challenge royal edicts" (Doyle, 2001, p.1) . These challenges often took the form of deliberate delaying tactics until the king held a lit de justice or sent a letter de cachet that would essentially force them to act. The parlements developed the power to pass arrets de reglement, which were laws that essentially applied within their jurisdiction.
So the Parlements were in fact part of the bedrock foundation of the Ancien Regime, and it was their wish to preserve that regime, with bourgeois, noble and royal privilege that may have led to its demise, at least in part. The parlements often prevented central authority (ie. the King) from carrying out miscellaneous reforms, such as changes to fairer forms of taxation. The ironic part of their attitude is that the parlements' refusal to allow these reforms actually challenged the very absolute power of monarchs that was at the basis of the ancien regime.
During the eighteenth century the parlements started to increasingly challenge the authority of King, ironically because he sought to change France. Thus they "frequently protested royal initiatives that they believed to threaten the traditional rights and liberties of the people . . . in widely distributed publications, they up the image of a historically free France and denounced the absolute rule of the crown that in their view threatened traditional liberties by imposing religious orthodoxy and new taxes" (Encarta, 2006) (my emphasis). The Parlements, while essentially conservative institutions in their wish not to change the precepts of the ancien regime, actually provided part of the energy that would lead to its downfall.
The Parlements did not act in isolation however. Thus their protests regarding Royal actions were joined by rich, but often progressive intellectuals called the philosophes. The philosophes did not advocate violent revolution any more than the Parlements did, but they helped the parlementaires in creating a system of thought and associated vocabulary that could be used to discuss and eventually plan the overthrow of an absolute ruler. The defined or redefined terms such as "despotism", "liberty", "rights" and "nation".
As Carlyle (2002, p.1.) puts it, in many ways the beginning of the end of the ancien reg