When a tick feeds, he buries his head under the surface of your dog's skin -- if you don't remove him properly, you can decapitate him and leave the head stuck in place. If you do, you may need to see your vet, as leaving the head under there can cause an infection. You must check the tick after removal to make sure that his head is still attached to his body.
Ways of Checking
Pulling the tick straight out in a firm and steady motion is the key to keeping his head attached to his body, but it isn't a guarantee -- sometimes, the head will detach despite your best efforts. The easiest way to tell if you got the tick's head out is to look at the body, because unless you squeeze it hard enough to pop the tick, it will be alive and wiggling its appendages. Its head will be visible at the end of the body. If you're not sure if you removed the head, take a close look at the bite spot -- the head will be visible as a black spot. Make an appointment with a veterinarian if you didn't remove the head, as leaving it in can result in infection.