There are a group of gum diseases that affects the periodontium (periodontium is made of the alveolar bone, the gums, the periodontal ligament and the cementum) at different levels.
The first stage is gingivitis: Gingivitis is inflammation of the gums on the surface and does not touch the alveolar bone. Gingivitis can be treated by a dentist along with having proper at home dental care habits. However when inflammation and infection reaches the perodontium in a deeper manner, the disease is called perodontitis.
Perodontitis has several stages which depend on how much tissue and alveolar bone is destroyed. The dentist is about to determine at what stage the periodontitis is by measuring the perodontal pockets (The space between a tooth and the gum around it) at six sites around each tooth. By doing this the dentist can get an idea of the destruction of the bone and the severity of the periodontitis