Answer:

There are plenty of sources on this of course, so the one thing I'll mention is that you will get a lot of answers saying Romantic music has melody and 20th century doesn't. This is inaccurate - Romantic music uses melody and melodic fragments as its main structural device, and so it seems emphasized, even omnipresent. In the 20th century, sound color, rhythm and sonority become more prominent , and the role of melody is reduced, but there is plenty of it still there.