This is the short story that launched Guy de Maupassant's career. Others have summarized it so I will try to do a bit of analysis. In the story of Maupassant is comparing three well-to-do couples, with two nuns and a prostitute who are sharing a ride in a carriage to a nearby town in Normandy. This is in 1870 when Prussian soldiers have invaded France.
Boule de Suif (ball of fat or tallow ball) is the way the prostitute is referred to in the story although her real name is Mademoiselle Elizabeth Rousset. She has been given that name because she is so fat. The carriage ride takes longer than expected and the passengers become very hungry. Boule de Suif is the only one who has had the forethought to bring a large hamper containing lots of food and wine. When they get to the next town, they hope to escape the Prussians they find. The town already occupied. The whole group stays at a hotel in the town and they will not be able to proceed until Boule de Suif agrees to have sex with the Prussian officer. She refuses to do so and the other passengers strategize on how they are pressured in order for them to continue their journey. She resists all of their arguments but finally gives in when one of the arguments does not mean that the end justifies the means. The nun says "An action reprehensible in itself often derives merit from the thought which inspires it." That evening, Cornudet, a member of the group, tells them "I tell you all, you have done an infamous thing!"
The officer then allows them all to continue on their way the next morning. The hypocritical passengers try to ignore Boule de Suif and one woman says, "What a mercy I am not sitting beside that creature!" Boule de Suif. They do not have any food. As they ride along tears from the eyes of Boule de Suif.
The attitudes of these people are not really that surprising. I have often found poor people to be more generous than well-to-do people.
The title "Boule de Suif" is an interesting one and obviously has symbolic meaning. It's an ugly image and yet it is the only one of the group that I would care to know personally. She is good-hearted which is more than can be said for the rest.
The character Cornudet understands what is happening and comments to the group, but also refuses to give Boule de Suif any food in the carriage . So in a way he is worse than the rest because he understands and still chooses to remain aloof.
de Maupassant never married and spent a great deal of his time with prostitutes. He eventually died in his 40s of syphilis. So this is a class of people that the author knew well.