(giri vs ninjo is a common theme of Kabuki plays)
GiriGiri (義理) is the Japanese sense of duty — the unwritten social contracts that ensure social harmony.
The classic example of giri is the responsibility that Samurai had to their masters. A Shogun could ask anything of a Samurai — even that a Samurai take his own life. Samurai were bound by giri to obey.
More modern giri relate to work, family and interpersonal relationships. For example, if you work at a coffee shop it's your duty to provide good service to customers. If your customer happens to be a jerk — your duty remains. Japanese society finds it distasteful when staff assert themselves with customers.
NinjoNinjo (人情) is the Japanese word for human emotion. In life (and fiction) ninjo often comes into conflict with giri.
The classic example, is the Samurai who falls in love with the Shogun's daughter. He is bound by duty to stay away from her. However, he's in love — his ninjo is running wild.
A more modern example could be a salary man who hates his job. He wants to quit to follow his dream of becoming a professional musician (ninjo). However, he has a family to support (giri).
A less dramatic example could be a waitress who has an obnoxious customer. Her giri tells to treat the customer with respect. Her ninjo tells her to spill a drink on him.
Young PeopleIn Japan, old timers complain that young people have forgotten about giri and are driven by ninjo. When giri declines and ninjo escalates — social harmony is threatened (according to conventional Japanese thought).
Giri ChocoloateOn valentines day, women in Japan buy chocolates for their boss, father and brothers. This is known as giri choko.
When a women buys chocolate for her boyfriend it's not considered giri choko. This gift is driven by her ninjo.