Anemia is characterized by low levels of hemoglobin or red blood cells. It can be caused by inadequate red blood cell production, increased destruction, or blood loss. Anemia is classified based on red blood cell morphology, etiology, or pathophysiology. Common types include iron deficiency anemia, megaloblastic anemia caused by vitamin B12 or folate deficiencies, and hemolytic anemias where red blood cells are destroyed faster than they can be replaced. Symptoms vary depending on severity but can include fatigue, paleness, and shortness of breath.