This document provides an introduction to intensive care units (ICUs), including definitions, classifications, and types. It defines ICUs as areas in hospitals that care for patients with life-threatening illnesses or injuries. ICUs can be classified based on staff qualifications, care complexity, and specialty. There are three levels of ICUs - Level I provides short-term care for less than 24 hours; Level II provides multisystem support in larger hospitals; Level III provides comprehensive long-term critical care. ICUs can also be open or closed based on physician management, and single-specialty or multidisciplinary based on patient types. High dependency units provide intermediate care and a step-down from the ICU.