What do worms eat depends on the species as some prefer as well as thrive better with different organic matter than others as well as where they reside.
Worms eat organic material in the soil, including fruits, vegetables, leaves and grass clippings
Since worms have small mouths and no teeth, they rely on Mother Nature to decompose organic matter by the way of microorganisms. Once the organic matter is decomposed sufficiently; worms can then suck the food in to be further grinded up through a gizzard similar to a chicken in order to eat.
Most all worms will eat decaying fruit and vegetable scraps such as lettuce, melon rinds, potatoes, celery, broccoli, apples… Some worms live towards the surface or come to the surface to feed. Other worms will remain below ground to eat.
Worms should not be fed meat or poultry products such as chicken, beef, eggs, cheese… Also worms should not be fed citrus foods such as lemons, oranges and even tomatoes. Other foods worms should not eat are onions and garlic.
Red Wiggler worms and Alabama Jumpers are the most proficient at eating food scraps.
Red Wigglers also eat and make an excellent cow, horse or rabbit manure worm. Another great manure worm however does not eat the regular food scraps as readily is the African Nightcrawler which also is an excellent fishing worm. African nightcrawlers are also extremely fond of damp shredded newspaper to eat which they can survive on, however will not be the healthiest of worms nor the largest if dependent only on the paper products.
Before shoveling manure to the worms to eat, be sure the animals producing the manure have not been given dewormer as this will kill off the worms.
Once the worms eat the decaying organic matter, they leave behind worm castings or worm poop. This is a prized organic soil activator and fertilizer by many gardeners as well as professional vineyards, golf courses and other commercial entities.
If raising worms, one food that is formulated especially for worms is Purina Worm Chow. It consists of grains as well as minerals and vitamins for the worms to eat. Purina Worm Chow may be given to your worms to eat as a main food source or as a food supplement. It also is great for fattening up worms for fishing as well as assisting in increasing the reproduction rate of worms since this diet produces larger and healthier worms.