The noun classic means something that's very high quality, particularly if it has lasting value. The pristine Model T Ford that you keep in your garage is a classic, but the beat up old Honda you drive everyday is not.
Use the adjective classic to describe something that relates to ancient Greece or Rome. You can enjoy the classic Greek pillars on the outside of your house but still appreciate the modern plumbing on the inside of it. Classic also means excellent, of recognized authority, or definitive. Classic literature, for example, includes authors like Mark Twain and Jane Austin.