Point of view is the perspective from which a story is told. If a character in the story is telling it, this is first-person point of view. This means we have an unreliable narrator because we only know the other characters through the narrator. We don't know if the character who tells the story is biased, giving us the events as he/she sees them and not how they really are. To tell if the story is first-person point of view, look for pronouns such as "I" and "we".
Third-person point of view is when the story is told by someone who isn't a character in the story. He/she is like an observer watching the story take place. The two kinds of third-person point of view are "limited" and "omniscient". Limited third-person is when the author allows the reader to know what only one of the characters is thinking or feeling. Omniscient third-person is when the author allows the reader to know what each character thinks and feels. You should look for pronouns such as "they" and "he/she".
Your question is therefore asking who is telling the story in The Adventures of Tom Sawyer. Is it a character in the story, or is it an observer who doesn't take part in the story? The point of view is third-person, limited omniscient. "This means that the story is told about Tom's world and is particularly focused on him by a narrator who is able to understand the motivations and feelings of some of the characters."