Most people know how to form the possessive singular of certain nouns. Just add 's. William's, Jesse's, the king's. But what happens when the noun already ends in s? This is surprisingly easy: just add 's. Charles's, the boss's, Mr. Evans's. If the noun is plural and ends in s, add only the apostrophe. The boys', the paraders'. This should be clear, yet people mistake the first of these rules, writing Charles' instead of Charles's. It's as if the extra s is somehow hideous to the ear. We already have one in there, it doesn't kill anyone to add a second.
Now that we have the rules out of the way, let's cover some exceptions. Ancient Greek names, such as Hippocrates, Socrates, and Euripides, use only the apostrophe. While it was Charles's book, it was Socrates' pen that wrote it. The biblical Jesus and Moses are also exceptions, though I have heard it suggested that, instead of writing Moses' laws, it would be better to write the laws of Moses. A stylistic preference, not a law.
Simple, right? Add 's to make a singular noun plural.