Until the 19th century, there was little distinctly American literature. Up until this time, writings were from the Puritans: those who recorded voyages or expeditions, preachers who wrote sermons, and poets who recorded personal experiences such as house burnings or other faith-challenging experiences. Historical recordings of the founders of the U.S. Constitution also contributed to the recordings of the human experience. Ben Franklin's Autobiography and Poor Richard's Almanac are among early American literature, as well.
In 1837 Ralph Waldo Emerson wrote,
We have listened too long to the courtly muses of Europe...The mind of this country, taught to aim at low objects, eats upon itself....We will walk 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, beacause each believes himself inspired by the Divine Soul which also inspires all men.
At a celebrated gathering in 1850, Nathaniel Hawthorne and Herman Melville determined that America would produce great writing. Perhaps because of the Puritan influence upon the new country, both Hawthorne and Melville discoverd a common bond in their insight into the dark side of human experience, becoming what was later termed Dark Romantics.
Ralph Waldo Emerson and Henry Waldo Thoreau, also produced some of the early masterpieces of American literature, masterpieces that focused on self-improvement and intellectual inquiry. Other authors such as Washington Irving became an Early American voice in his genius for inventing comic fictional narrators such as Diedrich Knickerbocker who narrates such folklore as "Rip van Winkle." Poetry issued from such early writers as William Cullen Bryant and Henry Wadsworth Longfellow, Oliver Wendell Holmes, and Emlly Dickinson. And, a writer who has had a powerful influence not only upon American writers but also upon latter European, and South American writers is Edgar Allan Poe.
These were some of the pioneers of American literature.
This is close to impossible to do. You must narrow your focus to themes in American literature, or to a particular genre or perhaps even to women in American lit. or even minority males in American lit. There is just too much to cover and too many eras of literature--puritanism, realism, naturalism, trancendentalism, modernism, postmodernism, etc., etc.
Here are the problems I find in your task. It would be difficult, if not impossible, to summarize in detail anything of any length in 500-600 words. Since American literature has continued to develop since the 1600s, summarizing it in detail would be impossible.
Then, I read the wording of your post more carefully. In saying "background," are you referring to the development/content of American literature, or are you really looking for a summary of the most important influences that produced American lit? This would be more "doable," but it still would be challenging.
Can you clarify your specific task in more detail?
Post #1 is correct in saying that you will have to narrow your topic or expand your word count, especially considering that anthologies abound on just one era from American Literature.
If you have to narrow down American Literature, you could do so by its theme--the American Dream. Almost all American authors address the idea of the American Dream directly or indirectly. In fact, what often determines an author's literary era is not only his time period but more importantly his view of the American Dream.
I think the question needs a more narrow focus. To summarize all of American Literature is an expansive task that will not reveal much different than any other nation's literature. Eventually, what will result is that the literature is about characters, thematic elements, and an ability to both reaffirm the current state of human beings or propose a demand to change those conditions. I think narrowing the topic down, or finding elements to provide a detailed summary about one period of American Literature might be something that can be more meaningful. For example, a summary of the Transcendentalists would consist of discussing their love of nature, favoring of emotions over reason, and the exploration of psychology in characters and self.
If the focus cannot be narrowed, I would suggest posting the question with more structure as to how it should be answered and this might involve opening a dialogue with the teacher because to summarize over 200 years of writing requires some parameters or boundaries.