In terms of history
It’s very basically “before Beethoven’s Fifth” and “after Beethoven’s Fifth.”
In terms of time spans
The classical period is said to run 1750–1825; the romantic period, 1825–1900. Romanticism would give way to Impressionism in 1890 and would overlap 20th century modernism (beginning in 1900).
In terms of large forms
Classical music gave us the symphony, solo concerto, sonata, and the string quartet. Romantic music saw the birth (or development) of the art song, symphonic poem, grand opera, and music drama.
In terms of musical characteristics
Classical music replaced the harpsichord with the piano. There was more of a distinction between orchestral and chamber music. Compositionally, more gradual crescendo/decrescendo (less terraced dynamics); more cadenza. Clearly defined formal schemes. Music tended more towards homophony. Melodies were often built on shorter melodic fragments (motives).
Romanic music often saw cyclical treatment of themes and/or theme transformation. More music was written for virtuoso instrumentalists. It was still largely homophonic. More rubato, as well as sudden and dramatic changes in mood and dynamic level. More use of motivic development. Rich harmony with more chromaticism. Bigger orchestras, longer compositions, new timbres. The piano became even more important.
In terms of composers
Chief classical composers would be Mozart, Haydn, Gluck, J.C. Bach, and C.P.E. Bach.
Romantic composers… wow. Just so many, but none more important the Beethoven.
German/Central Europe: Beethoven, Schubert, von Weber, Chopin, Mendelssohn, Schumann, Liszt, Brahms, Wagner, StraussFrench: Berlioz, Berlioz, FauréItalian: Verdi, PucciniEastern Europe/Russian: Tchaikovsky, Dvorâk, Mussorgsky, Borodin, Rimsky-Korsakov, Rachmaninoff, MahlerEnglish & Scandinavian: Elgar, Grieg, SibeliusFrom the inside back cover of The Understanding of Music, 6th edition (Charles R. Hoffer). Thanks to Han Lee for helping me straighten out some of the nationalities where I guessed wrong.
