All kinds of chocolate can make a dog sick. How sick depends on the kind of chocolate, the amount of chocolate, the size of the dog and the individual dog's sensitivity to it. Rule of thumb to remember: The darker the chocolate, the worse for the dog.
All chocolate contains theobromine, an alkaloid related to cocaine, nicotine, quinine and strychnine. Dogs cannot process this as rapidly as humans do and a big dose can overstimulate a dog's nervous system and cause fatal heart failure. The 12-ounce package of semi-sweet chocolate chips that goes into a cookie recipe could kill a small dog. Milk chocolate is less dangerous, but dogs vary in their reaction to theobromine and even a small amount could cause a strong reaction in a sensitive dog.
White chocolate actually isn't chocolate. It contains cocoa butter, but no cocoa solids. It also contains almost no theobromine. There's only 0.25 milligrams of the bad stuff in an ounce of white chocolate. It does, however, carry its own risks for a dog's health. It's loaded with sugar and fat, and eating it can upset a dog's stomach and even lead to pancreatitis. For these reasons, avoid commercial dog treats that contain white chocolate.
If your dog is a chocolate freak, there's actually a substance he can have that tastes vaguely chocolate-ish. It's also naturally sweet without the addition of sugar and low-calorie to boot. Completely harmless to dogs (and people), it's available commercially at health food stores and online. This miraculous substance is sometimes called locust bean or St. John's bread, but is mostly known as carob. It comes in powder form and can be added to homemade dog treats, or you can buy it in the form of chips to decorate those treats.