They are, yes. We are spirit and body, and when someone dies, their spirit enters the realm of Death. If there's a mage nearby, they can speed the spirits on their way to their final resting place in Death, but without the help they need to make the journey alone. Their spirits can become transformed, or twisted. More powerful, and more dangerous.
Necromancers can raise spirits, and bodies. Together or separately. The things they raise, we call the Dead.