Chocolate contains theobromine which is the toxic component. Humans easily metabolize theobromine, but dogs process it much more slowly, allowing it to build up to toxic levels in their system. The hazard of chocolate to your dog depends on the type of chocolate, the amount consumed and your dog's size. In large enough amounts, chocolate and cocoa products can kill your dog. Dark chocolate containing high concentrations of cocoa are the most dangerous, while milk chocolate in small amounts are more likely to cause gastrointestinal distress. With large amounts of theobromine ingestion, it can produce muscle tremors, seizures, an irregular heartbeat, internal bleeding or a heart attack. The onset of theobromine poisoning is usually marked by severe hyperactivity.
Xylitol (a common “sugar-free” sweetener in sugarless gum) is toxic to dogs because it's such a strong stimulator of insulin release in dogs. It takes just a small amount of xylitol (0.1g/kg) eaten by a dog to cause a dangerous drop in blood sugar ("hypoglycemia"). Mild hypoglycemia will typically cause weakness and a lack of coordination while severe hypoglycemia can cause death.
Grapes and Raisins are known to be highly toxic to dogs, though research has yet to pinpoint exactly which substance in the fruit causes this reaction. Because of that, peeled or seedless grapes should also be avoided. Ingesting the fruit could potentially lead to acute (sudden) kidney failure in dogs.
Onions and Garlic are members of the allium plant family. They contains thiosulfate, which is toxic to dogs but not to humans. Thiosulfate causes oxidative damage to red blood cells, resulting in hemolytic anemia. This decrease in red blood cells results in the cells not being able to carry oxygen to tissues in the body.