Apostrophe

The apostrophe ( ’ or ' ) is a punctuation mark, and sometimes a diacritic mark, in languages that use the Latin alphabet or certain other alphabets. In English it has two main functions: it marks omissions, and it assists in marking the possessives of all nouns and many pronouns. (In strictly limited cases, it is allowed to assist in marking plurals, but most authorities now disapprove of such usage; see below.) According to the OED, the word comes ultimately from Greek ἡ ἀπόστροφος [προσῳδία] (hē apóstrophos [prosōidía], "[the accent of] 'turning away', or elision"), through Latin and French.

The apostrophe is different from the closing single quotation mark (usually rendered identically but serving a quite different purpose), and from the similar-looking prime (which is used to indicate measurement in feet or arcminutes, and for various mathematical purposes).

Source: http://en.wikipedia.org/wiki/Apostrophe