Critics will say that the modernist movement in literature is actually best described as a period rather than an a cohesive artistic movement. However, we can certainly identify a number of common interests to the writers of the modernist period. It is this set of shared interests that has retroactively come to define the idea of literary modernism.
Modernist writing often emphasized formal experimentation as a means to express subjectivity and the variability of "truth" in narrative.
That humans are alienated from true knowledge of themselves is the implicit contention of the stream-of-consciousness form of narration.
Virginia Woolf and William Faulkner are two clear examples of these notions at work. Their novels are characterized by dense, highly original prose presented in conjunction within unconventional narrative structures, often using the stream-of-conscious narrative method.
The notion of subjectivity as the cornerstone of human experience was also an aspect of modernist poetry. Poets created new forms of narrative poetry, utilized broken structures and incorporating juxtaposition of ideas, feelings and languages.
...they attempted to model the fragmented nature of minds and civilization in their narratives.
T.S. Eliot, Ezra Pound and William Carlos Williams are examples of poets who are now considered to be modernist poets.
Beyond subjectivity of experience and ideas of the difficulty of truth, modernism demonstrated a personal relationship with the past that was often at odds with the growing trends of urbanization and industrialization. Themes relating to this emphasis on the individual's place in (and relationship to) history include alienation and Fruedian psychology.