1600 - 1750
Baroque music tends to be quite embellished, meaning the composer would usually write melody lines, and then the performers would add little flourishes as they kind of improvised on the melody. It is also very emotional and expressive, but unlike some later music, a Baroque piece typically focuses on a single emotion throughout its length.
1730 - 1820
Music from this era is largely diatonic, meaning that it has a definite key, and generally didn't stray outside it - though some might consider modulation the exception. However, the farther you go into the Classical Era, the more composers like Mozart, Haydn, and Beethoven would push the boundaries of diatonic and pull notes and chords from other keys, called chromaticism, a concept that the later Romantics would take to a whole new level.
1815 - 1910
Romantic music is characterized by two main things: emotion and nature. You'll find pieces that are rich with emotions so complex that you can't define them. There's pieces that are so simplistically beautiful they make you cry. And there's a number of works that were inspired by and invoke scenes of nature.