Classical usually refers to Greek and Roman antiquity — the classical period is said by many to run from the earliest Greek literature (Homer’s epic poems) through the closing days of the Roman Empire.
Sometimes classical is more specific, restricted to the period around the fifth and fourth centuries B.C.E. in Greece and around the first century C.E. in Rome, traditionally considered the high points of their culture. By this definition, very early authors (such as Homer and Hesiod) and late ones (Cassiodorus, Bede) are ancient but not classical.
Note: This guide is still in the early stages of development.
Three question marks mean I have to write more on the subject. Bear with me.