The apostrophe (‘ or ’) is a punctuation mark, and sometimes a diacritical mark, in languages that use the Latin alphabet and some other alphabets. In English, the apostrophe is used for three basic purposes:
The marking of the omission of one or more letters, e.g. the contraction of “do not” to “don’t”.
The marking of possessive case of nouns (as in “the eagle’s feathers”, “in one month’s time”, “at your parents’ [home]”).
The marking of plurals of individual characters, e.g. “p’s and q’s” or Oakland A’s.
The word “apostrophe” comes ultimately from Greek ἡ ἀπόστροφος [προσῳδία] (hē apóstrophos [prosōidía], ‘[the accent of] turning away or elision’), through Latin and French.
For use in computer systems, Unicode has code points for three different forms of apostrophe.
When the noun is a normal plural, with an added “s”, no extra “s” is added in the possessive; so “the neighbours’ garden” (there is more than one neighbour owning the garden) is standard rather than “the neighbours’s garden”.