Gingivitis occurs before periodontitis. Gingivitis usually refers to gum inflammation, initially, with gingivitis, bacteria plaque accumulates on the surface of the tooth. When that happens, the gums get red and inflamed; teeth may bleed when brushing them. Even though the gums are irritated and bothersome, the teeth are not loose, and there is no irreversible damage to bone or surrounding tissue.
Periodontitis is what happens next when Gingivitis goes untreated. With periodontitis, the gum and bone pulls away from the teeth, forming large pockets. Debris collects in the spaces between the gums and teeth, and infect the area. The immune system will start to attack the bacteria as the plaque spreads below the gum line. Bone and connective tissue that hold the tooth start to break down, and teeth over time become loose and can fall out.