Substantives in the English tongue, have only two terminations for case: the nominative, which merely expresses the name of the thing, and the possessive, or genitive case, made by the addition of s, with an apostrophe before it, which denotes property: all other realtions between substantives, as this also very often, are expressed by different prepositions: for example---a freeman's rights; or the rights of a freeman. When one thing is expressed as belonging to another, then one of the substantives is said to govern the other in the genitive, or possessive case; thus, Milton's poems; the king's gift; William's book. Here the former substantive, ending in s, marked with an apostrophe, is the word governed; as Milton's: the latter is the word that governs; as, poems: the former is in the genitive case, which is marked thus, 's, which mark is called the possessive 's.
Source: wiktionary