The harp is the only plucked-string instrument standard to the orchestra. Harps go back many thousands of years. Ancient Middle Eastern paintings show harps being played 2500 years before the Christian era. You can always identify King David in paintings and books because he was known to play the harp.
Small harps have been used by bards, minstrels and troubadors for many hundreds of years, because they are easy and portable. It was in the Renaissance in Europe that big floor harps began to be used in ensemble music. The hard part with harps was to make them so they could play in any key. They usually had only "white keys", and had to be retuned constantly. For a while, harps were built with two and three rows of strings, which made them quite hard to play.
It was in 1782, in France, that the "double-action harp" was invented. This innovation allowed the player to raise and lower the pitch of the strings using pedals. And it was from this method that the modern double-action pedal harp evolved. By the mid-1800s, there were so many double-action harps that Western composers were able to write orchestral parts for the harp. Tchaikovsky and Debussy wrote some of the loveliest harp parts.