As a general fights battles; as a ninja carries out espionage; as a monk converts other characters - all these things give a character experience points. These can be used to purchase useful skills, allowing you to shape your character's career path. The skill tree can be accessed through the button in the top right of any character details panel. Simply double-click on a character or right-click on their portrait to go to their details panel.
The Skill Tree
The skill tree is where the experience gained by generals and agents during a campaign can be used to acquire useful skills. As a character successfully carries out their functions, they gain experience points that gradually increase their ranking stars; each new star enabling multiple skill choices. Many skills also have several levels - each with increased effects - so it is possible to use experience to increase the level of a chosen skill, as opposed to selecting an entirely new one.