The document defines and describes key periodontal diseases: Chronic Periodontitis, Aggressive Periodontitis, and Necrotizing Ulcerative Periodontitis. Chronic Periodontitis is the most common form and involves plaque-induced inflammation that causes progressive bone and attachment loss. Aggressive Periodontitis affects otherwise healthy individuals and causes rapid destruction, while Necrotizing Ulcerative Periodontitis involves necrosis, ulceration and pain between teeth with potential systemic involvement. The document provides details on etiology, clinical features, classifications and prevalence for each disease.