Colloquialism

colloquialism: the use of slang or informalities in speech or writing. Colloquial expressions in writing include local and regional dialects and usage. Colloquialisms are to be avoided in formal writing; e.g. Jack was bummed out about his chemistry grade instead of Jack was upset about his chemistry grade

"'Who done it? We've heard considerable about these goings on, down in Hookerville, but we don't know who 'twas that killed Huck Finn.'"
                                                                -Adventures of Huckleberry Finn
                                                                           by Mark Twain

Twain uses the regional dialect here for multiple reasons. This story is told by a southern boy with next-to-no formal education; the use of slang and the writing in an accent gives the book the tone of a book written by an ill-educated boy. The reader also feels more comfortable and that they can relate to the boy, who speaks in the language they hear, and talk in. This makes the book a seriously moving book, as a campaign against slavery and injustice, and an enjoyable book. The use of the colloquialism also affords the reader the chance to experience life on the river, which many of the readers would never have known.