This is a disease of the gums. Plaque is material that forms on teeth that traps bacteria on the tooth causing it to breakdown and decay. At the base of the tooth hard deposits called tartar which also have bacteria trapped in will irritate the gums causing inflammation and infection.
Most everyone has some degree of gingivitis as it can be caused by illness, poor hygiene, hormonal chains or diabetes. If you brush your teeth hard enough to cause your gums to bleed or if you have braces there is a good chance that you may need to consider steps to reduce your gingivitis.
Teeth cleaning from your dentist will help remove plaque. Increased attention to oral hygiene and the use of antibacterial mouth wash will help gums heal and keep gingivitis from getting out of control and causing more serious periodontal disease.