i did not understand the usage of apostrophies.!!!

Apostrophy shows possession

Apostrophy used before 's' shows possession by one person
For example
Rahul's car - means the car belonging only to Rahul
Boy's bag - means bag belonging only to 'a boy'

Apostrophy used after 's' shows possession by two or more
For example
Sharmas' Party -It  means party organised by the entire Sharma family
Boys' Hostel - This usage lays stress on the fact that the hostel is only for the boys.