"But I don't want to spend that much money on food," you say!
Yes, I know. Me, neither. But you really do get what you pay for, and those grocery store kibbles generally aren't the best. And although many quality foods are more expensive, you really feed less because the ingredients are better absorbed by your dog or cat, and they aren't full of junk that just passes on through.
There are many different kinds of foods, and the more you are willing to pay, the better the product will most likely be. Decide what you can afford to spend on your pet's health, and find the best food you can with that amount. Find a "level" you feel comfortable in.
Generic: Some of the lowest quality brands are found in grocery stores or general stores and are loaded with junk. These generic foods, will often contain many allergens, a lot of fillers, poor quality proteins, and harmful preservatives. It's not uncommon for them to contain by-products, a lot of corn, and even meat sources that aren't specific like meat meal instead of a named protein source. These are typically foods that you feed a lot of, but that aren't absorbed well.
Poor: These are ones that have brand names you recognize and know, and often will trust. But many of these foods, too, have fillers, poor protein sources, and may have chemical preservatives as well.
Decent: Decent foods have natural preservatives, and have named protein sources. They will still generally have fillers such as corn or wheat, and will most likely contain some allergens. Overall, these are better foods and, in a pinch, I'd feed them. Most brands you'll find will fall into this level.
Good: These meats come from USDA inspected plants, and won't contain road kill, euthanized pets, rotten meat from grocery stores, etc. Some will have by-products, but they come from inspected facilities. The foods in this category will also use natural preservatives instead of BHA, BHT, or Ethoxyquin.
Great: Next are the great foods, or foods that have all the qualities of a good food without by-products. These will contain chicken meal or chicken as opposed to chicken by-product meal. They'll also have more meat sources listed in the first five ingredients than fillers like wheat and corn.
Excellent: These are human grade foods with no allergens. Common allergens are corn, wheat, dairy, and soy, to name a few. You'll see potatoes, barley, and rice in place of those other ingredients.
Top Notch: Then you finally come to the best kibbles you can find. These are excellent kibbles that have added probiotics, or digestive enzymes. They have chelated minerals, which are better absorbed by the body because they are bound to amino acids. And they have veggies in the mix, too.
Natural: Natural foods, which are foods you'd find your companion animals eating if they could choose to, is basically your food! Raw diets are becoming increasingly common, and more is being learned about this method of feeding. These can be purchased already prepared, or you can make them at home.