Asked and answered. Go to Literature/Modernism for a full discussion, and see the scholarship of Irving Howe on this subject. Thoroughly covered in previous answers. The text below is repetition for your convenience:
This term, which originally simply meant “art and literature of these times” was frozen into meaning the period approximately between 1850 and 1914, when the term “post-modernism” was coined. (It’s good to remember that these terms are coined after the fact, by critics and scholars, not by the actual artists.) It differentiated the current artistic impulses from the Victorian, 19 century, Industrial age; its primary features were a rejection of classical and Romantic influences in favor of modern (then) notions of Reason, human psychology, social equalities, and creative license; a re-examination of artistic forms such as free verse, nonlinear plots, and non-rigid forms; and a celebration of individuality vs. group identification. The term causes confusion because it embraces each movement as art progressed. Postmodernism refers to art that is about itself: self-conscious art, art that deals with its own forms as a subject--example: Unamuno’s novel, “The Fog (La Niebla),” in which a character in the novel goes to Unamuno’s house and asks not to be killed off in the novel. A central, seminal study by Irving Howe, “Modernism in Literature,” answers the question “What is Modernism” thoroughly.
Mainly fragmentation, estrangement and defamailirazation from standard conventions of literature.
Some modernist characteristics include: stream of consciousness, epiphany or the epiphanic moment, experimentation with literary form in all genres. Perception and knowledge - texts question what we see in the world, explore how we see them - also what we know in the world and how we know it