Twain used accents and slang words to bring his characters to life. Huckleberry Finn was different from anything most Americans had ever read. Twain wrote dialogue for his characters that made them sound like real people. He didn't make all his characters sound the same; instead they each had a unique voice.
Here is how Chapter One of The Adventures of Huckleberry Finn begins:
"You don't know about me without you have read a book by the name of The Adventures of Tom Sawyer; but that ain't no matter. That book was made by Mr. Mark Twain, and he told the truth, mainly. There was things which he stretched, but mainly he told the truth. ..."
Can you see how this captures the way someone speaks? This style of writing greatly influenced American literature, and The Adventures of Huckleberry Finn is now considered a classic novel, a work of excellence that is read long after it is written. Which classics have you read?