An apostrophe is a punctuation mark that looks like a tiny raised comma ('). It has two main jobs in English writing.

First, it shows possession, meaning that something belongs to someone or something. When you write Sarah's book, the apostrophe tells readers the book belongs to Sarah. For plural words ending in s, you usually just add the apostrophe after the s: the students' lockers means the lockers belonging to multiple students. This can get tricky: the children's playground needs an apostrophe before the s because “children” is already plural.

Second, apostrophes mark contractions, which are shortened forms of words. When you write don't, you're combining “do” and “not” and using the apostrophe to show where the letter o disappeared. Can't comes from “cannot,” it's means “it is” or “it has,” and they're means “they are.”

Here's a common mistake: its (without an apostrophe) means “belonging to it,” while it's (with an apostrophe) always means “it is” or “it has.” Think of its as working like “his” or “her,” which also don't use apostrophes for possession.

Apostrophes seem small, but they change meaning completely. “The dogs bowl” suggests dogs in a bowl, while “The dog's bowl” means the bowl belonging to one dog, and “The dogs' bowl” means the bowl belonging to multiple dogs.