Intensive Care Unit (ICU) and Intensive Therapy Unit (ITU) are synonymous.
An ICU offers 'Level 3' care, for "Patients requiring two or more organ support (or needing mechanical ventilation alone). Staffed with one nurse per patient and usually with a doctor present in the unit 24 hours per day." (reference)
An ICU is therefore used for patients with severe or life-threatening illnesses (particularly multi-organ failure) and injuries, requiring constant care and specialist equipment, monitoring, medications and interventions.