Case is a grammatical category which shows relation of the noun with other words in a sentence. The main meaning of the genitive case is that of possession. The genitive case may be marked, i.e. having an apostrophe s (’s) or unmarked, e.i. with an apostrophe only.
1) add ’s to singular nouns and names not ending in –s: Tom’s hat;
2) add ’s to singular nouns ending in -s: an actress’s career;
3) add ’s to irregular plural nouns: children’s games, sheep’s wool;
4) add an apostrophe (’) after the -s of regular plurals: the girls' teacher;
5) add ’s to proper names ending in -s: Charles’s car, Denis’s birthday, St. James’s Park;
Note: In many older, foreign and classical (ancient Greek and Roman) names, just add an apostrophe: Guy Fawkes’ night; Socrates’ wife; Cervantes’ Don Quixote; Euripides’ plays; Archimedes’ Law; Achilles’ heel.
6) add ’s to the final component of a compound noun: my mother-in-low’s dress;
7) add ’s to the last word in a phrase: The Duke of Norfolk’s sister; the Secretary of State’s private room; someone else’s gloves; in an hour two’s time; the boy on the left’s sister.
Possessives usually replace articles before nouns. We can say the car or Sue’s car but not Sue’s the car or the Sue’s car.
More about grammar: category of case.