A large group of musicians made up of four families of instruments: string, brass, woodwind, and percussion. Each section is led by a principal player; the principal first violinist is called the concertmaster.
The term orchestra comes from the name for the area in front of an ancient Greek stage reserved for the Greek chorus.
The orchestra grew in size throughout the eighteenth and nineteenth centuries, but changed very little in composition during the course of the twentieth century.
A smaller orchestra (of about forty players or less) is called a chamber orchestra.
A full size orchestra (about 100 players) may sometimes be called a “symphony orchestra” or “philharmonic orchestra”
A symphony orchestra will usually have over eighty musicians. A chamber orchestra could have as many as fifty members.
Resource: Wikipedia Encyclopedia on the Orchestra