Personification takes place when you imbue an object or abstraction with human characteristics. Done well, the descriptions that result can be unusually powerful. Here's an example from William Shakespeare's Romeo and Juliet: "Arise, fair sun, and kill the envious moon, / Who is already sick and pale with grief." Clearly, the human characteristics enhance not only the description of sun and moon, but also the relationship between the two, as well as the mood of the piece overall.
Here's another example, taken from The Masque of the Red Death by Edgar Allen Poe: "And now was acknowledged the presence of the Red Death. He had come like a thief in the night. And one by one dropped the revelers in the blood-bedewed halls of their revel, and died each in the despairing posture of his fall." The Red Death is, of course, pestilence, made all the more frightening by the implication that it lives, a thinking, plotting thing.