Dogs eat a variety of foods. However, domesticated dogs primarily eat dog food. There are many types of dog food out there. Most will have a recommendation for the type of size and breed of dog that you have.
Dogs are naturally omnivores, meaning that they eat both meat and plants. Most commercial dog foods consist of some form of meat (usually chicken, fish, beef, or a combination), grains (rice, barley, wheat, or a combination), and vegetables (carrots, corn, peas, or a combination).
Some dog owners feed their dogs once a day. Others do it per meal. Some owners leave food out for their dog all the time, and the dog simply eats when they are hungry. This is a personal preference. If you have questions about what is the best way to feed your dog, you can always ask your veterinarian.