The term symphony has two meanings. It is first and foremost a classical composition consisting of several movements in a somewhat rigid form; this form of music is where the term symphony originated and where its true meaning lies. But it has in more recent times come to refer to any type of classical orchestra; this meaning comes from the phrase symphony orchestra, which refers to an orchestra that specifically plays symphonies, but it is now used to denote any orchestra regardless of the musical forms they perform.
A symphony is distinguishable from other compositions by a series of elements that tend to define the form. The most important element of a traditional symphony is that it is written in four movements, no more and no less. The first movement is divided into the parts of sonata form; the second movement is slower and taken down in dynamic; the third movement is a more upbeat minuet variation on the second movement; and the last movement is a return to the first movement, with or without the sonata form. As the symphony evolved over time, many composers experimented with this idea of strict movements, often including more than four or altering the specific symphony rules within each. And though the traditional symphony is completely instrumental (another defining element), composers of later years began incorporating vocal melodies in complement to the various movements. This is a part of more modern symphony that is not incredibly common, however; most symphony orchestras are just that -- instrumental orchestras.
The first symphony was composed by Italian composer Giuseppe Torelli around 1698 and has since become the province of many infamous composers of several musical eras. Joseph Haydn was by far the most prolific composer of the symphony, composing over 100 in his lifetime. But it's Ludwig van Beethoven and Wolfgang Amadeus Mozart who frequently get credit for perfecting the symphony form. Though they may not have been main innovators of the symphony, they are remembered as being its most faithful composers.
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