The symphony is a large musical composition for orchestra which is usually written in four linked movements. The word Symphony is of Greek origin and is defined as "sounding together." The term was applied to various forms of ensemble music of the latter 16th century.
The symphony as we know it today however, is an 18th century development. It was then that composers implemented the elements of the Italian overture, merging the sonata and concerto grosso, a popular concerto of the Baroque period, into writings of the symphony.
As mentioned the structure of the symphony is written in four movements. The sonata is usually the first where three themes of two different but contrasting keys are presented. The structure is pertinent to the symphony as it was developed by composers of the accustomed Viennese style of writing, Beethoven, Haydn, and Mozart.
For the remainder of the 19th century into the 20th the symphony was a favored form for even more composers like Johannes Brahms, John Mendelssohn, Gustav Mahler and many more to come. I am certain that the symphony will be with us forever, just as many other popular among composers and performers alike, musical forms will.
'Symphony Number 5'
Composed by Ludwig von Beethoven
Symphony No. 41 'Jupiter'
Wofgang Amadeus Mozart