A symphony is an extended musical work written for performance by a large orchestra and often with a specific structure. Although there are many exceptions, symphonies traditionally consist of four movements. The first is lively and often in sonata form, the second is slower, the third is again lively, and the fourth is a dynamic finale which incorporates elements from the first three.
Musical compositions referred to as symphonies have been composed for hundreds of years and have evolved considerably. For example, the number and variety of instruments used in a performance has increased greatly, and a performance can now include more than a hundred players with more than two dozen different types of instruments. Although most symphonies are strictly instrumental, a few also contain vocal parts, most notably Beethoven's Ninth Symphony.
The word symphony can also refer to a symphony orchestra, which is a large orchestra that performs symphonies and other large-scale works, as well as to a performance by an orchestra.