A traditional concerto typically consists of three movements:
1. An opening fast movement in sonata form
2. A slow movement, often lyrical and in a contrasting key
3. A final fast movement, often featuring brilliant or virtuosic solo passages
On the other hand, a symphony traditionally has four movements:
1. An opening fast movement (Allegro)
2. A slow movement (Andante or Adagio)
3. A dance-like movement (Scherzo or Minuet)
4. A concluding fast movement (Allegro or Presto)
Therefore, the difference lies in the omission of a dance-like movement (Scherzo or Minuet) in a concerto. This is mainly due to the concerto's focus on highlighting the soloist's virtuosity and interaction with the orchestra.
The dance-like movement, common in symphonies, often provides thematic contrasts and a lighter character within the four-movement structure. In a concerto, however, the composer prioritizes showcasing the soloist's technical abilities and musical expression, leading to a streamlined three-movement format.