"Crusaders dominate battlefields through sheer purpose. The support of the heavenly Light and the sanctity of their quest lend Wrath to a crusader’s punishing attacks. As crusaders prepare for combat, their Wrath simmers, increasing slowly but steadily. When they cut down their foes, it boils over. A crusader overfull with Wrath is a dangerous adversary indeed: blessed with iron skin; swathed in scourging flames; blasting foes apart with bolts of radiant light, and springing back to life after defeat. Resourceful crusaders find means beyond dispatching enemies to fuel their Wrath. Their many skills allow them to recover spent resources through blocking attacks, suffering serious injuries, or when they’re encircled by snarling demons."— Game Guide(src)
Wrath is generated much like a Monk's Spirit in-game, being generated as the Crusader attacks. However, unlike Spirit, it also regenerates with considerable speed when out of combat, starting with an empty globe, much like the Barbarian's Fury. It is therefore a versatile resource that can be attuned to better accomodate the Crusader's tactics.
Initially the Crusader's Wrath is capped at 100, but it can be raised up to +30 more with items (+38 with Legendary items), as well as +30 with the Righteousness passive skill. Paragon levels may add up to +25 to this total. Wrath will innately recover at the rate of 2.5 points per second. This recovery can be increased from Crusader items by +6 (along with an additional +1.1 with one of Kormac's skills). The Crusader can also generate Wrath with Primary skills (+5 per attack, +3 more with Righteousness) or 6 per Block (with Insurmountable). The Provoke skill also generates 30 Wrath per cast, and generates an increasing amount depending on how many foes the Crusader provokes.