An Orchestra is a large group of musicians with traditions going back hundreds of years. There are four main families of instruments: Stings, Woodwind, Brass and Percussion. Sometimes orchestras perform with choirs or as accompaniment for soloists.
The wide variety of instruments means composers have a lot of scope when writing music to create many different sounds and textures.
You could argue that the origins of the orchestra lie in Ancient Greece, where plays were often accompanied by music, particularly choirs. The orchestra as we know it today originated in 1607 when the Italian composer Claudio Monteverdi wrote music for specific instruments with specific roles for each. Montiverdi is also believed to have been the founding figure of Opera.