Chocolate contains the chemical theobromine, which is toxic to dogs, as well as caffeine. While dark chocolate and baking chocolate has more of these substances than milk chocolate , no type or amount of chocolate is safe for a dog to ingest. It can cause death within 24 hours after the dog eats it.
As for fruit, some fruits are not only safe for dogs, but good for them. My dog loves pieces of ripe banana and apple. However, the seeds of fruits (apples, pears, etc.) contain arsenic, so they should be removed and thrown away before giving fruit to your dog. (The seeds aren't good for humans, either.) I peel, core and cut my dog's apple treats into bite-size pieces. Since apples can clean human teeth, they may be helpful for your canine friend's teeth after a meal. It's worth a try!
There are some other 'people' foods that are very dangerous for dogs and may be fatal. They include: Xylitol, an artificial sweetener found in chewing gum, candy, mints, toothpaste, mouthwash, and other products--READ LABELS; grapes and raisins; Macadamia nuts; avocados; coffee, tea or cola with caffeine; onions.
There is a lot of controversy over whether or not garlic is safe for dogs, and many people feed it to their dogs and swear that it's healthy. However, I've read on more than one veterinary website that a chemical in garlic (whether raw or cooked) can damage the red blood cells of dogs. They may not get noticeably sick after eating it, but once any damage occurs, affected red blood cells can rupture or lose their ability to effectively carry oxygen throughout the body. In my opinion, I'd rather avoid garlic for my dog than take a chance, however, slight it may be.
Aside from foods, there are many other substances and plants that are poisonous to dogs, so I encourage you to search out and read this information on veterinary websites. Dogs rely on us to keep them safe. It's the least we can do as loving pet guardians.