MarkTwain wrote humorous ironic stories that were embedded with satire. His aim always was to expose some one or more aspects of Western society that were not adhering to the moral and ethical code that Twain perceived America to be founded upon. For example, The Adventures of Huckleberry Finn and "The Celebrated Jumping Frog of Calaveras County" are satires.

Very distinctly American, Twain set the benchmark for American satire and parody.  (The scenes with the King and the Duke in The Adventures of Huckleberry Finn are classic.)  Will Rogers could not have had the schtick of folksy observations that he did without the irreverent Mark Twain.

I think above all, he was a social critic. Satire, of course, was part of that, but this is a man who, using one form of criticism or another, skewered religion, big business, racism, class inequality, imperialism, and just about every social ill of his time. He was by no means the first American to engage in this kind of criticism, but he was one of the first to appeal to a broad readership, and he was, in my opinion, the best at it.

In addition to all of the above, Twain's works can be included in the genre of realism. He depicted America, particularly the South, as it truly was, which is why it can be painful for some people to accept. Though he did write in satire and he exaggerated many aspects of the areas, he was not too far off from the truth. The views of whites toward blacks were real. People may be offended by his writing, but he depicted things as they truly were without holding back for fear of making people angry.

Twain was also a master writer of dialects. The voices of his characters ring absolutely true in the expressions, structure of spoken thought, and pronunciations that would be heard if you were eavesdropping on conversations instead of reading words on a page. Much of the humor, satire, and honest observation of human nature that Twain expressed in his writing comes out of the mouths of the people populating his writings.

There is one word that you need to understand to unlock the greatness of Twain's work, and that is satire. Twain was above all a master of satire, or the ability to mirror various faults and vices of society and individuals back to that society in order to try and promote change. The way that Twain uses irony and exaggeration to achieve this is nothing short of brilliant. You might like to consider The Adventures of Huckleberry Finn and how Twain satirises Christians and also society as a whole.

Twain is usually remembered as perhaps the greatest humorous writer in all of American literature.  That's how he was mainly valued in his day, and that's how he tends to be valued today as well.  To say this is not to imply that Twain could not be profoundly serious when he wanted to be (the chapter in Huckleberry Finn in which Jim recalls how he slapped his young daughter is truly heart-breaking), but there are few writers of any nation who makes us laugh so much and so often as Twain does.

I think his genre should simply be called American literature.  You could argue that Huck Finn or Tom Sawyer are juvenile fiction, but I think that they are entertaining and insightful even for older people.  I would call Twain's stuff American literature simply because it is very difficult to imagine the sorts of books he wrote being written by anyone but an American with an American's sense of the need for democracy and an egalitarian society.

Twain's "stuff"?