In a previous post, I discussed American Literature's "crisis of representation."  I think that the second half of the question is an equally fascinating one.  The last twenty years has seen an increase in the number of voices entering the pantheon of American Literature.  I don't see it as a twisted quota system, whereby we are "plinking" off groups that have been previously silenced.  Rather, in high schools, universities, and (yes, even middle schools), we are seeing learned people strive to make literature more relevant to the voices of the audiences, who either bring different experiences to the classroom or, through greater social interaction, are cognizant of different experiences.  I think that works like Morrison's Beloved are examples of how the African- American predicament is becoming a more standard part of the curriculum.  Women's studies and the analysis of the predicament of women in America are becoming critical components of how we define American Literature.  The fact that American Literature recognizes that Native American literature can speak to different tribal experiences indicates how expansive the genre has become. The one area that might need further mention is the realm of sexual orientation.  I would say that the works of bell hooks, writing as a lesbian, a woman, and as someone from a different level of class than the traditional author, and Tony Kushner's Angels in America are but two examples of a growing voice emerging in the dialogue of gay, lesbian, and transgendered individuals.  These are a few examples of how the genre has grown, matured, and expanded in scope and depth.

When the English critic Sydney Smith (1818) declared.

Literature the Americans have none...In the four quarters of the globe, who reads an American book?

he ignited a reaction from the Transcendentalist Ralph Waldo Emerson, who wrote in "The American Scholar" in 1837,

We have listened too long to the courtly uses of Europe...Wht mind of this country, taught to aim at low objects, eats upon itself....We will wlk on our own feet; we will work with our own hands; we will speak our own minds....A nation of men will for the first time exist, because each believes himself inspired by the Divine Soul which also inspires all men.

What, then, is American literature?  It is a celebration of this independence of thought, a celebration of the strength of those who have fought for liberty, political or social or spiritual.

The first American narratives were written by Washington Irving, who related tales based upon local legends. Of course, the Transcendentalists, of whom Emerson was one, perceived the importance of intuition and the strength of the individual and doorway of Nature to truth; they had intense optimism about America.  After them came Edgar Allan Poe, who formed a counterpoint to the optimism of the Transcendentalists along with Nathaniel Hawthorne and Herman Melville who, too, perceived the dark side of human nature.  But, the common thread through all of their writings is emphasis on the individual.

One European-American woman who wrote highly lauded novels about individuals is Willa Cather whose 1927 "Death Comes for the Archbishop"  is based upon the real lives of Archbishop Lamy, the first bishop of New Mexico, and his vicar, Father Joseph Macbeuf both of whom emigrated from France.  Another novel of Cather's, "My Antonia," is the story of both Antonia Shimerda, a Bohemian immigrant who came to Nebraska, and the novel's American-born narrator, Jim Burden.

The portrayal of Antonia is recognized as one of the most memorable of the twentieth-century as, through this character, Cather celebrates the pioneering spirit of immigrants new to American. 

Since the previous posters have defined American literature so well, and offered examples of African-American literature, I'll give some of female and Native-American writers.

Women like Edith Wharton (The Age of Innocence, Ethan Frome), Willa Cather (My Antonia), Shirley Jackson (The House on Haunted Hill, We Have Always Lived in the Castle, The Lottery) and Sylvia Plath (The Bell Jar), just to name a few, have shaped the nature of American literature from the 18th century onward.

Native-American writers have been just as prolific. Leslie Marmon Silko is known for her beautiful and brutally realistic portrayals of life on the Laguna Pueblo reservation. Sherman Alexie, a member of the Spokane-Coeur D'Alene tribe, has written numerous novels, lyrical poetry, and several books for children. His work combines the traditions of his heritage with the realities of living in contemporary times.

There are so many more that I didn't mention, all of whom have contributed something great to American literature.

American literature truly is a national tapestry that has grown through the centuries to encompass and recognize writers from all cultures that have contributed to the American experience. Some of the earliest writings of the Tewa people, for instance, are now included in American literature anthologies. One of the most beautiful reflections of the Tewa culture is "Song of the Sky Loom," a poem rich in figurative language.

American literature was enormously enriched by the works of African-American writers who came out of the Harlem Renaissance of the 1920s and by those who followed in their literary footsteps. Langston Hughes, Zora Neale Hurston, Claude McKay, Countee Cullen, and Nella Larsen of the Renaissance were followed by new, powerful African-American writers, such as Lorraine Hansberry and Alice Walker.

And the lists go on. American writers of so many different cultural backgrounds have made our literature especially rich and representative of the nation as a whole, then and now.

I went very basic to start off in this question:

American literature refers to written or literary work produced in the area of the United States and Colonial America.  During its early history, America was a series of British colonies on the eastern coast of the present-day United States. Therefore, its literary tradition begins as linked to the broader tradition of English literature. However, unique American characteristics and the breadth of its production usually now cause it to be considered a separate path and tradition.

This site address is featured below.  I think there should be some expansion in its explanation, though.   American Literature, like all literature, has one foot in literary experiences and the other in the contextual history of America.  It depicts how America has changed, similar to a blackboard that has been erased over and over.  Like most canons of literary works, what constitutes as American Literature has changed over time.  Three or four decades ago, "American Literature" was defined narrowly in scope and did suffer from a "crisis in representation" of different voices in the American pantheon.  To a large extent that has changed, and the definition of American Literature has come to embrace works by women and hyphenated Americans.  In a shortened essence, we can look and examine American Literature, like America, itself:  An experiment of multiple voices, constantly changing to add to its own rich and challenged history and mutable identity.