The apostrophe is not just for contractions. There's a misconception that "John's car" is really a contraction of "John his car", but that's not really the case. According to Wikipedia, its use actually originates from people
mistakingly thinking that.
The most difficult part (aside from all the crazy things mentioned on Wikipedia) is the syntax for words that end with an s, especially plural versions. Here you have to simply follow what sounds right (since not using an extra s is also syntactically correct). Charles's car, Dickens's books, Marx's theories, but: the glasses' contents, the Williamses' house, the buses' passengers.