The Feeder is extremely skinny and its torso is grotesquely emaciated, with its bones and skeletal system clearly visible. This allows them to have increased agility and lets them jump from various platforms and move quickly. A majority of the Feeder's head reveals the skull of the human host as all cartilage, flesh and skin has been removed while its jaw is permanently held wide open. The Feeder's hands have been distorted into bloody talons used for hacking and slashing their victim.
A Game Informer interview revealed that Feeders are formed from humans ingesting Necromorph flesh. What would lead an individual to perform such a gruesome act will most likely be revealed during the game. The likely hypothesis is starving survivors that need food.
The most likely reason for a human to ingest Necromorph flesh would either be starvation or wanting to become "one" with the Necromorphs, the latter being most likely out of insanity or being a devout Unitologist.
According to a Game Informer interview, Feeders will not attack unless provoked or shining light on them.