Before the conquest of England by the Duke of Normandy in 1066, the language of the country was what we call Old English. This change in leadership changed nearly everything else in the country, including the literature.
Old English literature is characterized by great tales of valor about heroic figures who eventually fall. These stories were composed and sung long before they were formally written down, and they contained elements of mythology. The most prevalent theme is that the destiny of man is determined by fate (Wyrd), and great acts of heroism would be followed by a great downfall. The most well known and representative literary work of this period is, of course, Beowulf, which declares “Fate will unwind as it must!”
Middle English literature is distinguished by the French influence after the Norman invasion. The language used in writing was Latin. English had many dialects including the dialect used in London where Chaucer lived. Literature after the Conquest was written in French or Latin until Chaucer. Geoffrey Chaucer was often criticized for writing his Canterbury Tales in the dialect of London (which much later became "English") rather than in French or Latin (but his choice was popular as this work was the most widely reproduced work of the period, with many original copies still in existence).
In addition to the changes in language, Middle English literature also changed in theme. The Church, in the eleventh century, wanted to reduce the violence and created a common enemy. They chose the heretic Saracens (Muslims), and thus the Medieval knight was born in life and in literature.
A knight from this period embodies the virtuous qualities supported by (though not necessarily practiced by) the Church. He is a selfless man who protects virtuous women and the weak; he is willing to fight the enemy of all for the benefit of all. Literature such as the Song of Roland led to all the grand tales of courtly love: a knight's platonic love for a lady which prompts him to do great deeds in her honor. These stories are called romances.
Perhaps the most representative, or at least the most popular, Medieval romances are the King Arthur stories and Sir Gawain and the Green Knight. It was said of Gawain:
Gawain was reputed as virtuous, like refined gold,
Devoid of all vice, and with all courtly virtues
What Old English and Middle English literature have in common, at least for a time, is the poetic form; however, many of the same themes from both periods still appear in literature today.