The exact time periods are a matter of dispute; but generally I've seen it as being called the "Dark Ages" or the "Early Middle Ages" from the fall of the Roman Empire, the source of the time disputes - say 476 - until the 10th century and the Middle Ages proper being the period after the 10th century, with an arbitrary date using Hastings in 1066 or Manzikert in 1071 (for some reason MTW uses the ascension of William II to the throne of England in 1087). The ending date is in dispute as well; generally sometime in the late 15th century, using various event (Columbus and the New World, the final stages of the Reconquista, Gutenburg's printing, or as in MTW, the end of the 100 Years War and the Turkish conquest of Constantinople, both in 1453)

Petrach coined the term Dark Ages. Today it is used more to delineate a period lacking in written records, roughly 400-1000. Sometimes they add a transition period and to include the less "dark" aspects in Byzantium and the Arabic world called "Late Antiquity" instead. And some authors date the beginning as the advent of Mohammed and so use the 7th century.