For those who want to know where the apostrophe came from, here is how it probably happened. Some of this is well documented, some is guesswork on my part. Back in the days when English had many more inflections than it now has, the most common suffix for the genitive singular was -es. The apostrophe stands for the omitted letter. Pronouns were also inflected, but not in the same way.
Plural nouns are harder to explain.
The apostrophe in these cases normally has no effect on pronunciation. Thus dogs, dog's, and dogs' all sound the same. This is, however, a fine and almost inaudible distinction. Using an apostrophe in a plural which is not a possessive form is almost never recommended by prescriptivists. Be careful in these cases to put the apostrophe in the correct place.