What role did St. Thomas Aquinas, the Franciscans and Dominicans, and the popular heresies play? My text for this is Traditions and Encounters by Bentley and Ziegler.
The people and groups that you mention played very different roles in medieval European society. St. Thomas Aquinas was more of an intellectual while the Dominicans were more concerned with everyday life. The popular heresies were ideas that gave comfort to the people, but which were opposed by people like the Dominicans.
St. Thomas Aquinas was not a man who was mainly concerned with helping the poor or converting souls on an individual basis. Instead, his role was among the elites of society. He wanted to bring Christian faith and rational thinking into harmony with one another. This is not something that tended to matter to the average person of Aquinas’s time. Saint Thomas tried to explain how Christianity and the thought of Aristotle (seen as the greatest of the ancient thinkers) could be reconciled with one another.
The common people of the time did not care about such things. They wanted to have a religion that could help them make sense of their world and which might help them better cope with that world. This was the role of the popular heresies. For example, Catharism taught that there was a good God that created all things spiritual and an evil god that created all things physical. This could help people get their minds around the idea that God loved them even as their physical lives were so hard. The Dominicans came in to the picture because they wanted to save (as they saw it) people from the popular heresies. They were not intellectuals in the mold of Aquinas. Instead, they simply wanted to get out among the people and propagate the true faith (again, as they saw it).
Thus, these people and ideas/groups played different roles in Medieval European society.