Classical Period pieces distinguished themselves from the earlier Baroque style and its long melodies by their sudden changes of mood in quick succession. This would happen instantly or gradually, changing the theme, or even including changes within the theme. Fluctuations in mood were something new compared to the unbroken rhythm of art music of the previous 150 years. In addition, frequently used contrasting themes made thematic dualism a core element of the Classical Period structure.
Another distinction between the Baroque style and the Classical Period is the homophonic texture of the latter, as opposed to the basically polyphonic Baroque. In simple words, this means that Baroque pieces included multiple melodic voices that remained independent, while Classical music incorporated a prominent melody with other melodies accompanying it or acting as a background. However, homophonic texture was not an inviolate rule, as pieces could change textures, starting as homophonic and afterward becoming polyphonic.
Classical composers put great emphasis on structural clarity, creating easy-to-remember melodies as contrasted with the complicated Baroque tunes. Classical Period melodies are characterized by their symmetry and balance, consisting mainly of two equally long phrases. The term phrase refers to the grouping of melodic notes, which, according to William Mathews on his work "How to Understand Music," is "a passage of melody that makes sense, but not complete sense."
The invention of the piano in the late 17th century was especially important for Classical Period composers, who needed an instrument that could allow for soft and loud notes. Since piano strings were hit by a hammer, instead of being plucked -- as was the case with the harpsichord -- players could vary their finger pressure to produce the appropriate notes. This was a major help during gradual changes in Classical pieces -- the crescendo, when the tune becomes louder, and diminuendo, when the tune grows softer.