Some creatures, such as Trolls, are able to regenerate damage their flesh immediately healing from the most serious of wounds. This rule confers a special regeneration save to the model which works exactly like a 4+ ward save, with the following exceptions:
Wounds suffered from a flaming attack cannot be regenerated - no regeneration save is allowed against such attacks.
Regeneration saves are not ward saves, and can therefore be taken after armour saves and ward saves have been failed!
For example, a Von Carstein Vampire Count has a an armour save of 5+ (due to his barded steed). He is also wearing the infamous Carsten Ring, which gives him a ward save of 4+ and the Regeneration special rule. If the Vampire is wounded by a crossbow shot (-1 armour save modifier), he will take his modified armour save of G+ first. If this is failed, he will then take his Ward save of 4+, and if this is failed too, he will take a regeneration save of 4+ (as the crossbow is not a flaming attack). He's really hard to kill!