What are the themes and characteristics of Modernism?

It's hard to establish exactly when Modernism began, but most scholars argue that the Modernist period began in the late 19th century and reached the end of its height by World War II. Some of Modernism's most famous authors include Ernest Hemingway, Virginia Woolf, James Joyce, Ezra Pound, William Faulkner, T.S. Eliot, Gertrude Stein, and F. Scott Fitzgerald.
In many ways, Modernism was a response to the influx of the revolutionary ideas of thinkers such as Darwin, Freud, and Nietzsche. Altogether, these individuals presented ideas that deconstructed important traditional foundations of society, such as religion, ethics, sexuality, and even the identity of the individual. Additionally, the technological advancements and chaos of World War I caused the widespread doubt that humanity was actually progressing; indeed, after the unprecedented violence of the war, many people began to believe that human society was descending into turmoil and meaninglessness. 
The Modernist movement began in the capitals of Europe, and it was concerned with exploring themes of alienation and dislocation in society, and with the struggle to find meaning and identity in a rapidly changing world. Artistic techniques of the period include Impressionism, Cubism, stream of consciousness narrative and more, and, through these techniques, artists sought to explore the fragmentary, suddenly messy nature of modern life. In general, much of Modernism grapples with the sense that, with the loss of traditional values, some vital aspect of human society was also lost. As such, much of the art and literature produced during this period incorporates the sensation of wandering and searching for some kind of value. It's hardly surprising, therefore, that Stein referred to some of the most prominent Modernists as "the lost generation."