The book which belongs to one boy is the boy's book. The book which belongs to the group of boys is the boys' book. Notice in the case of the singular possessor of the item, the apostrophe is on the inside, i.e., between the noun 'boy' and the possessive marker 's'. In the case of the plural possessor, it is on the outside.

Possessive pronouns (e.g., mine, yours, his hers, ours, theirs), if they require a possessive marker, do not take an apostrophe, so the book belongs to my sisters, the book is theirs. In the case of mine, the possessive is implied and so you don't need to add the 's'. It is therefore incorrect to go bragging after you get the ring that the man is mines.

Compound words are sometimes tricky. The car belonging to my sister-in-law is my sister-in-law's car and the statement from the board of directors is the board of directors' statement. When there is joint possession, the possessive marker is only applied to the final noun in the group, e.g., the house belonging to my mother and father is my mother and father's house, not my mother's and father's house.