Every time a dog or cat eats, small particles stick to the teeth and combine with bacteria to form a sticky film called plaque. If this plaque is not cleaned off, it will harden into a hard, brown layer called tartar. Tartar is a semi-permanent substance bonded to the tooth itself that harbors dangerous bacteria that begin attacking the gums and the roots of the teeth. This is periodontal disease, and is an irreversible process.
The Veterinary Oral Health Council has a very informative website explaining periodontal disease and preventative oral care.