A number of rules that define how monsters work in the Worlds of Adventure campaign are different from Pathfinder.
Most creatures fall into one or more categories or (a.k.a. “types”), such as “dragon”, “humanoid”, “fire creature”, “living creature”, and so forth. A creature may belong to any combination of categories that applies (though some categories may be incompatible, such as “living creature” and “undead creature”). Much like spell types, these categories usually have no intrinsic game-mechanical effect, but affect how various spells and abilities interact with different sorts of creatures: a ranger’s favored enemy ability, the effects of positive and negative energy, the charm person? spell, etc.
Most creatures can make combat reactions to dodge attacks, but not to block. A creature treats its hit dice as its warrior level for the purpose of determining its dodge value.
Some creatures (those that are proficient with weapons and armor, and have a “martial” orientation, such as hobgoblins or fire giants) can block attacks. Other creatures (those whose abilities are focused entirely on things other than martial combat, such as spell weavers) can neither dodge nor block. (Such creatures may be able to take the Dodge feat to gain the ability to dodge attacks.)
See combat reactions for more information on dodging and blocking.
Some types of creatures in the Worlds of Adventure campaign do not take extra damage from critical hits (nor, by extension, from abilities like a thief’s sudden strike, the Deadly Stroke feat, and other abilities which function like critical hits). Magic abilites which activate on critical hits (such as as a flaming burst weapon) still work against such creatures, if the ability does not depend on the creature having a differentiable anatomy, weak spots, etc.
Creatures are most commonly immune to the extra damage from critical hits due to having a body which is an undifferentiated mass of matter, or due to not having a physical body at all. The most commonly encountered kinds of creatures that fall into this category are:
This is not an exclusive list. In general, a monster which has no part which is any more important to its functioning than any other part is immune to the extra damage from a critical hit.
Some monsters may have “class levels”, granting them abilities very much like those gained by characters who are members of character classes. This may come about in various ways: some monsters may study, train, and practice various skills and abilities much like a character of the common races might; other monsters may have the abilities associated with this or that character class simply by virtue of their nature.
Monsters with class levels do not, by default, gain hit dice or level-dependent benefits (base attack bonus, save bonuses, skills, and so forth) from class levels. Thus a storm giant who has the abilities of a 10th-level barbarian still has the same basic statistics and combat numbers as a normal storm giant — but has the ability to enter a rage; a dragon with levels in the cleric class can cast spells and turn undead, but has as many hit points, etc., as a normal dragon of its age; etc. (The exception to this is a monster who gains class levels in excess of its racial hit dice. One way of seeing this is to think of a monster as having its race as its primary class progression, and any class levels it might take as a secondary progression.) Some monsters may even have some, but not all, of the class abilities of one or more character classes (this is often the case for those monsters that gain character class abilities by virtue of their nature, instead of through specific personal experience or training).