The apostrophe is the most commonly misused punctuation mark. There are two uses for apostrophes:

Contractions are words where one or more letter has been omitted: the apostrophe is used to indicate that letters have been removed. Some of the most common examples are:

Contractions can be used in formal writing, but you should use them only rarely as they make writing more chatty.

Some words feature an apostrophe because they were once contracted, but now we only ever use the contracted form:

Possessives indicate that something belongs to someone. The place of the apostrophe depends on whether the possessor is singular or plural. A singular possessive goes before the possessive 's'; a plural possessive goes after the possessive 's'.

Something to remember: children and people are plurals, but plurals that don't end in 's' should have a possessive apostrophe placed before the possessive 's'

N. B. the possessive 'its'

The possessive 'its' causes endless confusion with apostrophes. ‘Its’ is the possessive form of ‘it’:

Like 'his' and 'hers', when being used in the possessive sense ‘its’ never takes an apostrophe. It takes an apostrophe only when it's a contraction.

Another common mistake is to put an apostrophe in plain plurals i.e. plurals that just indicate more than one. Plain plurals never take apostrophes:

The incorrect use of apostrophes irritates markers, mainly because it's quite easy to get them right.