In vague technical terms, this is a French realist story about a young disaffected impoverished officer who learns the mechanisms of social norms, and uses them to advance his own position within society, with substantial commentary on the decay of French morality in the 19th century. 
 In other words, it's the story of a complete shit-heel who manipulates money and people and rises through society quite easily. Maupassant has a few cutting remarks (see - fencing scene) and he has a very nice contrast between the nice descriptive scenes of the countryside, and the utterly disgusting behavior of not only Georges 'Bag-o'-Dicks' Duroy, but all the rest of the characters too. 
 I'd only read Maupassant's short stories before, but it's good to see his condensed descriptions work well when extended to a novella in length.