In some ways, music from the Romantic era was an expansion of what was happening in previous eras, like the Classical and Baroque eras.
The Classical period was guided by structure, and composers of the Romantic era began to bend or break the "rules" that governed music in the Classical period. Many Romantic composers, such as Mendelssohn, Brahms and Schubert, followed a more Classical style, while other composers, such as Berlioz and Wagner, pushed the boundaries. Romanticism was furthered by impressionist composers such as Debussy.
The Classical period used a rigid tonal structure.
Composers in the Romantic era began to stray away from this rigidity and used more diminished and half-diminished sevenths, Neapolitan sixths and augmented triads. What was considered dissonant in the Classical period became more consonant in the Romantic period. Composers used more chromaticism and modulated to keys unrelated to the tonic key, so tonality itself began to crumble.
Form and Style
Romantic composers used traditional styles and even added older elements from the Baroque period, such as polyphony; however, they did not feel restricted to maintaining the form and made sections longer.
Techniques like the idée fixe and leitmotiv became popular. Music in the Romantic era also became more programmatic. Music became nationalistic by composers using styles and forms of the folk music of their native countries, and music was more personal. Many new forms arose in the Romantic era, including the tone poem, song cycle, nocturne, impromptu, intermezzo and polonaise.
Freedom of Expression
Freedom of style and expression was more evident in the Romantic era than the Classical period.
Melodies became more disconnected and had wider ranges. Rhythms became more complex, and composers used more syncopation -- changing meters and complex meters. Music also had more thematic material. Composers felt free to break the rules of form, harmony and melody.