Virtue

Typically used with: Self-Control.

In short: abstain. Those who strengthen the Virtue skill are effectively able to resist temptation. While it doesn't necessarily represent a strong moral code, it does infer a character has strong force of will over their impulses.

Difficulty

Most of the time, Virtue checks are opposed by attempts by other characters and creatures to lure, entice, or enrage. Otherwise, a character who makes a Virtue check is resisting general temptation or compulsion (e.g. a kleptomaniac tries to resist stealing, a recovering alcoholic resists a drink. The difficulty of the check should follow the guidelines at the beginning of the Skills chapter.

Opposed Rolls

Virtue checks are opposed by Seduce and Taunt checks, as well as some spells and similar powers.