When Huck tells a tall tale about why he was "delayed" on his way to Aunt Sally's house he describes a river boat accident in which no one dies, only an African American. This little detail, while arguably satirical, demonstrates the prevailing attitudes regarding the relative value of life according to racial distinctions.
Since Mark Twain himself scorned the institution of slavery, he used it as his vehicle for satire of several characters such as Pap [who comes to mind in relation to the character of Bob Ewell in To Kill a Mockingbird], Miss Watson, the King and the Duke, and even Tom Sawyer. Twain implodes the ideas about slaves not being people who experience emotion and love and kindness. For, some of the most poignant scenes involve Jim as he cries when he believes that Huck is lost. After Huck does return to the raft, Jim cries and hugs him as a father would, opening Huck's mind to the very humanness of the "negro."
At the beginning of the book, Huck shares the viewpoint of the others in his surroundings - blacks are slaves, whites are superior. As time goes by, we see his perspective change as experiences and observations allow him to acquire insight into Jim's personality, actions, motivations, and conduct. Huck comes to realize that Jim is very similar to himself, which means that he can't be racist based on a person's skin color.
As the first post mentions, the book satirizes racism. The more Huck gets to know Jim as a person, the less Huck can accept the racism of his society. The same evolution is, ideally, true of the reader as well. A strong case can be made that Jim is the true hero of the book -- that is, the character who, by the end of the novel, most commands the reader's respect.
There are many people who misunderstand the book, and think it is a horrible, racist book. They point to the repeated use of the n word, the portrayal of slavery, and so on. Are you researching the presentation of racism, or how people interperet it today?
This novel takes a satirical view toward racism in the South. It is told from the point of view of a young white boy who has been raised to have certain attitudes toward African Americans. The racist views of Huck and of the rest of the white people in the area are highlighted and exaggerated (though it's not much of an exaggeration really) to show how outrageous the racist view are.
Pap is a good example of the view toward racism in this novel. He is a drunk and an abusive father; what one would consider the most low-down kind of person there is. In theory, he should be at the very bottom of the social ladder because all he does is drink and does not have a job. However, he tells of when he goes north, and mentions a free black man that he sees there. Pap is outraged at the behavior of the black man because the black man has more class than he does. Pap thinks that just because he is white, he is above a black man no matter what other factors come into it.