A hospital management system ER diagram visually represents the structure and relationships within a hospital database. It helps in organizing and managing critical information such as patient records, doctor details, appointments, treatments, billing, and departments. The main entities in the ER diagram include patients, doctors, nurses, appointments, wards, treatments, and administrative staff. Each entity is connected through relationships that define how data flows within the system. For example, a patient entity is linked to the appointment entity to record which doctor the patient meets and the date of consultation, while the doctor entity connects with the department entity to identify their specialization. Similarly, billing is associated with the patient and treatment entities to maintain accurate financial records.