A template is a set of rules that you apply to a monster to transform it into a different monster. All templates give precise directions on how to change a monster's statistics to transform it into the new monster.
This kind of template is added to a creature well after its birth or creation.
Some templates, like the lich, are the results of a creature's choice and desire to transform. Others, like the ghost template, are the result of an external force acting upon a creature (for example, when a tormented person dies and becomes a ghost). Yet in both cases, the template changed a creature well after its birth or creation—these types are called “acquired templates,” and can be added to a creature at any time during its existence.
Some templates, such as the half-dragon and half-fiend templates, are part of a creature from the beginning of its existence. These types of templates are called “inherited templates.”
Creatures are born or created with these templates already in place, and have never known life without them.
Simple templates can be applied during the game with minimal effort. This makes it easy, for example, to deal with celestial and fiendish creatures conjured in the heat of battle by summon monster spells.
All simple templates have two categories of changes.
The “quick rules” present a fast way to modify die rolls made in play to simulate the template's effects without actually rebuilding the stat block—this method works great for summoned creatures.
The “rebuild rules” list the exact changes you make to the base stat block if you have the time to completely rebuild it—this method works best when you have time during game preparation to build full stat blocks. The two methods result in creatures of similar, if not identical, abilities.