Nature created cats to be carnivores. That means that they eat meat, and have little to no use for grains in their diets. In Nature when they eat their prey, they are eating the wholesome muscle meat , bones of the animal or bird.
When you're looking for a cat food then, you need to find one what doesn't contain a grain as the first couple of ingredients (corn especially), and one that contains no by-products. These aren't meat - they are the scraps parts that have been deemed unfit for human consumption. Such bits as beaks, hides, tendons, spinal chords, intestines, spleens, diseased meat, tumors, spoiled meet - even the scrapings of the slaughterhouse floor - go in to by-products. Not exactly the good muscle meat that your cat would choose to eat.
Good cat foods contain very few grains, and no by-products. Bad cat foods contain corn or other grains as one of the first couple ingredients, and by-products. Avoid anything that says Maise,grains, Animal derivatives/by-products (this is the scrapings off the factory floor!)
The best food for your cat is all wet or raw.
Dry food is now linked to such feline disorders as obesity, diabetes, urinary tract disorders, and kidney disease. Dry food was created for our convenience, and not for our cat's health.
In nature cats don't drink much - their moisture requirements are met by eating their live prey. Good quality Wet food is as close as you're going to get to that without feeding a raw diet.