In a classification essay, your job is to present your claim by placing a topic in a category that it might not always be placed in. For instance, some people might not put "pigs" in the pet category. However, if you wanted to make the argument that pigs make great pets, you could show the categories into which pigs could be placed alongside traditional pets such as dogs and cats (such as the ability to be housebroken, affectionate toward humans, and ability to learn tricks).

Here is a resource that will give you more information on writing a classification essay (as opposed to division-- or analysis).