NPC is short for Non Player Characters. Theses are controlled by an AI.
You will find NPCs in every port, there's are a few different types.
Theres NPCs are part of the Missions System in game. When a NPC have a mission that is avaiable to you a "!" appears above their head. A "?" above means that they are part of one of your current missions.
Pirates only have Skill trainers for Fighting skills as they can not take part in careers like other Nations can. The Fighting NPCs are, Florentine Trainer, Fencing Trainer, and Dirty Fighting Trainer
Each Nations, each Faction has a NPCs ships on the open sail. If your faction standing with that is low enought, often they may attack you.
Attacking NPC ships nears ports effects the Contentions points of that port.