Middle English (ME) is the name given to the English language roughly from the 11th to the 15th centuries: after the Norman invasion by William the Conqueror in 1066 and before the introduction of the printing press. It was the language of the peasant and the butcher, not of the king and the nobles, who spoke French. It was the language of Geoffrey Chaucer.
English before 1000 is called Old English or Anglo-Saxon; after 1500 comes the era of modern English.
History
1000
Syððan wæs geworden þæt he ferde þurh þa ceastre and þæt castel: godes rice prediciende and bodiende. and hi twelfe mid. And sume wif þe wæron gehælede of awyrgdum gastum: and untrumnessum: seo magdalenisce maria ofþære seofan deoflu uteodon: and iohanna chuzan wif herodes gerefan: and susanna and manega oðre þe him of hyra spedum þenedon;
-- Translation of Luke 8.1-3 from the New Testament
When the Vikings conquered England, they had also conquered northern France and became gallicized (as in English they became anglicized). In 1066, led by William the Conqueror, these gallicized Vikings, the Normans, attacked, conquered, and ruled England (and still ruled northern France). The country became trilingual: French became the language of the king and the nobles, Latin the language of the priest and professor, and English the language of the people.
This profoundly changed the English language.
Even now, after a thousand years, the Norman class-system is still visible in English: the words for common things are derived from Old English, for example: pig, cow, dog and house.
The words for things used by the rich and the ruling class are derived from middle French, for example: pork, beef, court, judge, jury, parliament, honor, courage, rich and study.
Even the word for the less wealthy classes came from the mouth of the francophone: poor.
The triplicate vocabulary of English comes from this Norman period. For instance, English has three words meaning roughly "of or relating to a king": kingly from Old English, royal from French and regal from Latin. Each carries its own nuance.
The Old English kingly brings to mind a fabled king; the French royal, the ample pomp of a medieval court; and the Latin regal, the noble expression and manner of a king, an abstract king.
Deeper changes occurred in the grammar. Bit by bit, the wealthy and the government anglicized again. But the new English didn't look the same as the old. Old English, just as any good Indo-European language of its day, contained many grammatical endings. But in the mouth of the French-speaker, the endings were lost and forgotten, the English broken and rough, using word order instead of the proper endings. "Incorrect language", but still understandable, and above all, a simplified language. And, because the re-anglicized nobleman had wealth, power, and high rank, his English wasn't mocked but instead became the English language.
The language also simplified because during the centuries of French dominance: English-speakers stood outside the educated class, that class which could brake rapid, rough change.
1300
After standardization of the language, English began to appear almost in its modern form:
And it is don, aftirward Jesus made iourne bi cites & castelis prechende & euangelisende þe rewme of god, & twelue wiþ hym & summe wymmen þat weren helid of wicke spiritis & sicnesses, marie þat is clepid maudeleyn, of whom seuene deuelis wenten out & Jone þe wif off chusi procuratour of eroude, & susanne & manye oþere þat mynystreden to hym of her facultes
-- Luke 8.1-3
As can be seen from this, Middle English is quite close to Modern English. However, there are some important differences. Person is shown using -st for 2nd singular, -eþ for plural and -s for 3rd singular. No ending is used for subjunctive. There are also more weak masculine -en plurals in Middle English, such as eyen (eyes). See English third person for an explanation of third-person suffix differences. (There are other differences. We need them explained.)
The vast differences between Old English and Middle English have led some to claim English is a glorified creole. See Is Middle English a Pidgin? for a discussion.
In the 14th century, English finally became the language of the government. This was the century of the great poet Geoffrey Chaucer.