As the title suggests these are Huck Finn's adventures although Tom Sawyer has some part in them at the beginning and at the end. After becoming rich at the end of Tom Sawyer Huck Finn's father reappears and wants to get his hands on the money while Huck is officially living with the widow Watson. Although the father doesn't get the money he kidnaps Huck and forces him to live with him and locks him up. Huck Finn escapes on the Missisippi and makes it look like he has been murdered so noone comes looking for him. After some time in hiding he is joined by Miss Watson's slave Jim who ran away. The two start a journey down the Missisippi on a raft together.
The book is devided into several parts marking different locations and stories which could be looked at seperately for each has significance regarding one or more aspects of American life in the South at the time. Huckleberry Finn is always right in the middle of the story but usually not the person responsible for the occurring problems. That doesn't mean that he's not capable of changing the course of events. Although he is a vagabond and often has to lie and trick people he has got a conscience and always wants to do the right thing. The most important conflict he is in is his friendship with Jim. Huck knows that Jim is a good person and they have to rely on each other to survive. But being brought up in the south he is supposed to see Jim as property, as a slave, not as a person. One of the most impressive passages of the book is Huck Finn pondering about giving Jim up or saving him to freedom. It's not an easy decision because there are laws, morals and religion weighing against his own feelings and conscience. Needless to say he does the right thing - as Twain does too at the end of the book in setting Jim free officially (the widow died and gave him his freedom in her will).
It wasn't easy to read Huck Finn as a non-native speaker of English. Mark Twain explains:
In this book a number of dialects are used, to wit: the Missouri negro dialect; the extremest form of the backwoods Southwestern dialect; the ordinary "Pike County" dialect; and four modified varieties of this last. The shadings have not been done in a haphazard fashion, or by guesswork; but painstakingly, and with the trustworthy guidance and support of personal familiarity with these several forms of speech.Spelling and sentence structure were hard getting used to especially when Jim was talking but it was worth it because I guess a just translation would be nearly impossible. This kind of dialect writing was new at the time and Twain worked hard to get there, rewriting the text several times. So it must be important for depicting the situation at that time.
I make this explanation for the reason that without it many readers would suppose that all these characters were trying to talk alike and not succeeding.
I won't get into all the problematic interpretations and controversies (for being racist or anti-racist) that Huck Finn raised over the years and still does, e.g. eliminating the book from the reading lists (because of the use of the word "nigger") as a symbol of change after Obama's election into office (see wikipedia article on the topic).
The most impressive about the novel is its narrator's voice and protagonist. In addition to that it is remarkable how it touches so many sensitive topics of American history without being judgemental but showing clearly that there were things that were problematic on different levels. That makes it an outstanding novel at the time it was published (1884) and today.