Historians define the Medieval period in Europe as the intervening years between the 5th to 15th centuries. As for the state of England during the Middles Ages, things can be split relatively concretely into two time periods: before the Norman conquest, and after.
The Medieval period began in earnest with the fall of the Western Roman Empire. It was a cultural disaster. The subsequent vacuum led to mass social and political upheaval throughout the continent-- years of famine and poverty from which we get the term "dark ages".
In the years immediately following the collapse of Rome, England was controlled by the Anglo-Saxons, who were the descendants of Germanic tribes who had emigrated to England and adopted Christianity. The years of their rule saw the island develop a strong centralized nation-state, complete with robust economy and armed forces.
Saxon control of England ended, though, in 1066. William I (or William the Conqueror) led a Norman invasion, overthrowing Anglo-Saxon rule and introducing a new, Fuedal model of social organization. Most of the population became serfs-- peasants bound to work land owned by their local lord. Meanwhile the King and other nobles controlled the populace through taxation and force. This system was manifest physically in a country-wide network of castles.
Eventually, the Middles Ages transitioned into the Early-Modern period, following dramatic advancements in science, culture, and science which improved quality of life for much of the continent.
It's interesting to note that, although we sometimes refer to the Medieval era as "the Dark Ages", contemporary scholars didn't think that way at all. Medieval writers referred to their own time as "modern". Which is funny, but also somehow enlightening. Humans, throughout history, have a tendency to see their own lives as the most important act in the play. It can pay to keep this in mind when considering our own world. We, like the people of Medieval England, are just another chapter in the long book of human history.