Apostrophes are used for two things:
To show a contraction (i.e. that one or more words have been left out), e.g. He can’t eat me now. (Contractive apostrophe)
To show possession, e.g. The crocodile’s teeth were rotten and twisted. (Possessive apostrophe). Note:
To show possession on singular noun that already ends in s, you can use either of these two forms, e.g. James’ and James’s.
But a plural noun already ending in s only takes an apostrophe, e.g. all of the soccer players’ injuries or both ladies’ cars.
Extra information:
Possessive pronouns do not use a possessive apostrophe, e.g. Which house is his? and That crocodile is furious. Its eyes are rolling. (It’s only uses an apostrophe when it’s a contraction, e.g. It’s a furious crocodile [It is a furious crocodile].)