An orchestra is an instrumental ensemble, usually a fairly large instrumental ensemble with string, brass, woodwind sections and possibly a percussion section as well.

A full size orchestra (about 104 players) may sometimes be called a "symphony orchestra" or "philharmonic orchestra".  A symphony orchestra will usually have over eighty musicians on its roster, in some cases over a hundred, but the actual number of musicians employed in a particular performance may vary according to the work being played, and the size of the venue.
A smaller orchestra (of about forty players) is called a chamber orchestra. A leading chamber orchestra might employ as many as fifty members; some are much smaller than that.

Wikipedia "Orchestra"

Phoebe O'Leary

In a more modern usage, a symphony or symphony orchestra is an orchestra, particularly one that plays or is equipped to play symphonies. Going to hear a symphony orchestra play is sometimes called "going to the symphony," whether or not an actual symphony is on the programme. - Wikipedia at: http://en.wikipedia.org/wiki/Symphony#Symphony_as_.22orchestra.22 Amy Sample Ward