As I discussed in an earlier post on Magna Carta (promulgated in 1215), English Parliament had nothing to do with the "great charter" during the Middle Ages. Parliament came along three generations later.
But today Britain's Parliament and Magna Carta are both considered to have something to do with "English liberties," so one can see why they are often run together in people's minds.
So where did Parliament come from? The word is from the Old French, indicating a place or gathering where people talk (parler) and discuss. It had long been established in all of medieval Europe, going back to the beginning of the Middle Ages, that kings were not supposed to make unilateral decisions. If they did, they were tyrants. Rather, they were supposed to work through councils made up of the powerful men (and very occasionally women) of the realm.
In England, kings who knew what was good for them called councils of the powerful whenever they had something important to decide. At different times different collections of people might be asked to come. These powerful people were considered to represent the populace and nation as a whole. Such councils were called parliaments during the thirteenth century.
In 1295 a parliament was called which was later described as the "model parliament." The people who were summoned then to advise the king became the model of who was supposed to be called. No one at the time thought they were "founding Parliament," but the next few parliaments summoned the same people, and it became a tradition.
The "model parliament" had two wealthy burgers from each town and two knights from each shire, as well as all the powerful dukes and counts and the bishops and abbots of the realm. The towns represented in 1295 continued to be the towns represented in Parliament until the nineteenth century, when changes had to be made—some major cities had grown up which had not existed in 1295, and other medieval cities had shrunk so much that they scarcely had more than the two people needed as representatives.
During the thirteenth century the Parliament settled down to have two "houses," the House of Lords, made up both of the great aristocrats and of the church leaders, and the House of Commons, made up of the burgers and knights of the shire. The modern British Parliament still has these two houses, although the Commons has had almost all the power for the last couple centuries, the reverse of the medieval situation. Parliament long had judicial as well as legislative functions; Britain got a separate Supreme Court only in 2009.
Early in the fourteenth century, a generation or two after the model parliament, both France and Spain developed similarly institutionalized forms of councils to advise the king. In France the Estates General persisted (although frequently ignored by the kings) until the French Revolution. Interestingly, the French had three "estates" rather than England's two "houses."
In France, the three estates were the church (separated out from powerful lay lords), the nobility (which included knights who would have been in the Commons in England), and wealthy townspeople. France's regions also continued to have "parlements" in the medieval and early modern periods, regional assemblies which were primarily judicial.
The above image is of the Houses of Parliament in Westminster (what Americans would think of as part of London). It is not medieval.