A narcissist is someone who cares only for themselves, termed after a Greek mythological character, Narcissus. The story goes that he's was so obsessed with himself that he falls in love with his reflection. That is the cornerstone of a narcissistic personality. They are so self-absorbed that they lack the basic human trait of empathy (being able to put themselves into someone else's shoes).
Without empathy, you are left with a person who use everyone in their awaken moment as a means for their own pleasure. In fact, people in a narcissist's world only exist for the sole purpose of pleasing themselves.