Modernism is not a well-defined artistic movement, but as it relates to literature modernism can be described in at least a few ways. Post-modernism is equally loose in its definitions, in fact more-so than modernism.
The simplest description or definition of literary modernism refers to the time in which the modernism movement took place. The narrowest time-oriented definition of modernism places this movement between the two World Wars, from 1917 to 1937. Writers like Faulkner and Hemingway, Pound and Eliot, Joyce and Woolf were all working at this time and are all associated with modernism.
If we look at subject, theme and style to define modernism, we can say that this literary movement was marked by a personal relationship to the past, to history, and also inclined to explore the subjective nature of perception (and, by extension, reality).
Post-modernism, which again is a less clearly defined than modernism, can be described as a generalized movement toward broken narratives, self-contained and self-referential works, utilizing fantasy elements more than had been used previously. Some writers whose works are sometimes called post-modern: Thomas Pynchon; John Ashbery; Margaret Atwood; Roberto Bolano; Toni Morrison.