Middle English is the language which Chaucer spoke, and in which he wrote.

Middle English throve from the twelfth century to the fifteenth century, and represents the gradual amalgamation of Old English with Norman French following the Norman Invasion in 1066. During that time, several characteristic Old English letters drifted out of the alphabet:
thorn þ, Þ "th" sound (unvoiced)
eth ð Ð different "th" sound :) (voiced)
ash æ Æ; good question

There's also something called wynn, which looks a lot like thorn and seems to languish unrepresented in ISO 8859-1Latin-1. wynn signified some kind of "w" sound.

Middle English differed in vocabulary and sentence structure from Modern English. for example, they'd do subject-object-verb as in "I hym folwed", or verb-object-subject as in "taughte me my dame". Both examples are from Chaucer. Much of the vocabulary is recognizable, but "distorted"; of course, it's our own late modern English words which are "distorted" descendants of those words, not the reverse. Either way, you can recognize "folwed" as "followed" if you've got your wits about you.

The term "Old English" is horrifically misused. The language of Shakespeare and of the King James Version of the Bible -- the two were contemporaneous -- is early Modern English. Neither Shakespeare nor Chaucer ever wrote in Old English. The odds are about 10,000 to 1 (if not steeper) that what you call "Old English" is early Modern English.