Although plural nouns that end in s take only an apostrophe, singular nouns that end in s take an apostrophe + s – the boss's office, James's dog, Lois's friends, Mr. Jones's car. One exception, though, is that we use only an apostrophe when the word formed by that apostrophe + s is awkward to pronounce – that is, when adding an "iz" or "ziz" sound difficult. For instance, "Jesus(iz) teachings," "Moses(iz) laws," or "Sophocles(iz) philosophy" would fall into this category. Therefore, we write: Jesus' teachings, Moses' laws, and Sophocles' plays. Actually, in such instances, it is better to sidestep the possessive case entirely by writing "the teachings of Jesus," "the laws of Moses," and "the plays of Sophocles."
For a more detailed discussion of possessive nouns, see Possessive Nouns: The Apostrophe in the Writers' Guide section.