Fact is there are lots of people who can't tell the difference between a guard dog and an attack dog. To be fair, at first glance it seems that the word “guard dog” and “attack dog” are one and the same thing. Guard dogs are trained to attack when threatened; likewise attack dogs will stand guard until given the command to attack.
But they are actually two very different things. It's not just what they do, but it's also about the training these dogs go through to become what they are. Comparing a guard dog and an attack dog is, simply put, like comparing apples and star apples.
If you're looking for the obvious difference, consider their owners and trainers.
A common owner of a guard dog is the homeowner and civilian. Many people feel safe knowing there's a dog in their house, and many people keep dogs in their property for this reason alone. Richer homeowners go the extra mile by sending their dogs off to guard dog training schools to become better at what they do: guarding unattended houses and property.
Attack dogs are found elsewhere. Law enforcement officers and other members of the force use attack dogs to track evidences, sniff through luggages, chase criminals, etc. Being an attack dog is more difficult and requires a lot more discipline on the dog's part, all things considered.
Guard dogs go through guard dog training. Guard dog training is often performed in school. It is possible though to train a guard dog at home. A dog may begin its guard dog training as an adult, a characteristic that attack dog training doesn't seem to share.
Attack dogs become what they are through attack dog training. Training an attack dog is an intensive procedure, and for that it needs to begin as early as possible in the dog's life. In fact it's safe to say that proper attack dog training is a lifelong process – for the dog, that is.