Informal, Youthful, Colloquial
Since Huck is our first-person narrator, the whole story is told in his voice. And boy is it distinctive:
I didn't want to go back no more. I had stopped cussing, because the widow didn't like it; but now I took to it again because pap hadn't no objections... But by-and-by pap got too handy with his hick'ry, and I could't stand it. I was all over with welts. He got to going away so much, too, and locking me in. Once he locked me in and was gone three days. It was dreadful lonesome. (6)
The grammar isn't perfect, and Twain writes the way Huck Finn talks (hence all the apostrophes subbing for unpronounced letters). And you can tell by the way he talks about pap's abuse—that he got "too handy with his hick'ry"—that he just accepts the beatings as a part of life. Behind the colloquial and friendly tone, it's actually kind of sad.
Besides nailing Huck's education level, social background, and personality, Twain succeeded in telling the story convincingly through the eyes of a thirteen-year-old. (At least, we think so.) The novel drips with dramatic irony, when we can pick up on certain subtext even when Huck doesn't. Like all those conversations where Huck thinks he's fooling somebody into believing one of his lies?