Most people divide birds between softbills and seed-eaters, but this does not adequately describe the real variety of dietary habits. Softbill is a confusing term because it is so broad, and there are also various types of seed diet.
Starting with seeds...these are divided between cereal seeds (a favorite of finches) and oil-based seeds such as sunflower seeds. In addition to this you can get seed mixes for a specific species such as parrot mix. The important thing to note with something like parrot mix is that this alone will not meet all the dietary requirements of your pet bird. Some examples of seeds and mixes are: softbill food, spray millet, budgerigar seed, canary seed, blue maw, parrot pellets, niger seed, parrot food, and foreign finch seed.
Onto the softbills...the diets of other birds can be classified as carnivorous, insectivorous, omnivorous, frugivorous, nectarivorous, and folivorous. Let us look at what each of these diets consists of. The carnivorous diet is made up of mammals such as rodents and other birds. Insectivorous birds eat insects such as flys. The omnivores have a mixed diet of plants and food. Frugivorous is a less well known terms and simply refers to those birds that eat fruit. Finally nectarivorous birds eat flower nectar, and foliovorous birds eat plants.
Diet is something that bird owners need to carefully research, not only in terms of required foods, but also those that are dangerous. For example, the theobromine found in chocolate can kills birds because they metabolize it more slowly than humans. Avocado is also toxic to birds. In terms of drinks coffee and alcohol are not suitable so you must clear these drinks from sight when your bird is out of its cage for exercising. Foods that are high in salts are also unsuitable for birds.
Finally a word on fruit as part of the bird diet...
Fruit is a common addition to the diet of a pet bird, but it is important to ensure that it is carefully washed or peeled to remove any harmful pesticides. You will soon find out which fruits your bird enjoys, with grapes a common favorite. If you have a tropical bird like a parrot and it is refusing to eat fruit you can try warming it briefly in the microwave to bring it to a jungle temperature. Another important tip with fruit is not to provide too many different kinds of fruit as this can upset the stomach.