The apostrophe serves three functions: contraction, plural and possessives.
It’s = it is
Avoid the most common mistake: “its” is the possessive form of it.
It is rare for an apostrophe to be used for a plural noun.
The Jones’s have been extinct since the 1980’s.
The Joneses have been extinct since the 1980s.
Certain abbreviations or words are exception to the rule.
He scored four A’s and three B’s.
Unless the apostrophe is needed to avoid confusion or misreading, leave it out.
*Source: The Chicago Manual of Style, 16th edition. Newspapers use Associated Press style for possessives.
Singular noun: add apostrophe and s.
The kid’s room
Plural noun: add apostrophe and s unless the noun already ends in s.
The kids’ toys
Use only an apostrophe for places or names that are singular but have a final word in plural form and ending with an s.
The United States’ immigrant problem
Nouns that end in an s sound take only an apostrophe when they are followed by sake.
For goodness’ sake