Who or what is "the devil" in The Midwife's Apprentice? What do the devil's visits reveal about the villagers?

At the time when the story is set—the Middle Ages—belief in the literal existence of the devil was widespread. It's no exaggeration to say that medieval folk believed that Satan was everywhere, actively engaged in a constant battle to corrupt humankind and lead men and women from the path of righteousness.
Belief in the devil hardened into a dangerous superstition, which encouraged people to ascribe Satanic qualities to people they didn't like. Women who didn't fit in, for example, or who appeared strange to their neighbors in the village, were often demonized as witches.
In The Midwife's Apprentice, Alyce uses this superstition to her own advantage, getting back at the villagers for all the hatred, scorn, and suspicion that they subjected her to. She comes up with the novel idea of carving hoof prints into planks of wood, which she then stomps into the ground leading up to the houses and meeting places of the people who've wronged her.
She knows that the superstitious villagers will take one look at the hoof prints and conclude that the devil's been sniffing around the village, looking to take the souls of sinners. The hoof prints are too small to be a horse or a cow, they figure (there are also only two of them). So they conclude that the prints must've been left by the devil.
Alyce's mischief leads to some pretty embarrassing situations for the village's resident sinners. The devil's footprints lead the priest and some of the villagers to the forest, where they find the baker preparing for an illicit rendezvous with Jane the midwife. The baker's wife is accompanying the priest on his late night devil-searching expedition, and it's fair to say she's not pleased with what her husband's been up to. Contrary to what she might believe, however, her husband's sins have not been revealed by the devil but by Alyce.