An orchestra ( or US: ; Italian: [or'kestra]) is a large instrumental ensemble typical of classical music, which mixes instruments from different families, including bowed string instruments such as violin, viola, cello and double bass, as well as brass, woodwinds, and percussion instruments, each grouped in sections. Other instruments such as the piano and celesta may sometimes appear in a fifth keyboard section or may stand alone, as may the concert harp and, for performances of some modern compositions, electronic instruments. The term orchestra derives from the Greek orkhestra (orchestra), the name for the area in front of a stage in ancient Greek theatre reserved for the Greek chorus. A full-size orchestra may sometimes be called a symphony orchestra or philharmonic orchestra.
Instrumentation
The typical symphony orchestra consists of four groups of related musical instruments called the woodwinds, brass, percussion, and strings (violin, viola, cello and double bass). Other instruments such as the piano and celesta may sometimes be grouped into a fifth section such as a keyboard section or may stand alone, as may the concert harp and electric and electronic instruments. The orchestra, depending on the size, contains almost all of the standard instruments in each group.  In the history of the orchestra, its instrumentation has been expanded over time, often agreed to have been standardized by the classical period and Ludwig van Beethoven's influence on the classical model. In the 20th century, new repertory demands expanded the instrumentation of the orchestra, resulting in a flexible use of the classical-model instruments and newly developed electric and electronic instruments in various combinations.  The terms symphony orchestra and philharmonic orchestra may be used to distinguish different ensembles from the same locality, such as the London Symphony Orchestra and the London 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.  Chamber orchestra usually refers to smaller-sized ensembles; a major chamber orchestra might employ as many as fifty musicians; some are much smaller than that. The term concert orchestra may also be used, as in the BBC Concert Orchestra and the RTE Concert Orchestra.
What instruments do they play in an orchestra?
four groups of related musical instruments called the woodwinds, brass, percussion, and strings
four groups of related musical instruments called the woodwinds, brass, percussion, and strings