In a nutshell, ICU is where patients go if they are very sick. The floor (medsurg) provides basic nursing care. ICU nurses need to be well versed in caring for critical patients, titrating vaso-active drips and working a kazillion pieces of equipment. However, they generally only have a couple of patients to care for. The Med-surg nurse has to be adept in caring for many patients, frequent admissions and discharges, and more assisting the patient with ADLs.They really are night and day. I would rather care for a coding patient, than have 6 or 7 "stable, floor" patients.
Did you not rotate through an ICU in school? ICU is not floor nursing. Floor nursing can be called medsurg, surgical, medical, ortho, neuro, GYN the list goes on. It is basically for stable patients. The names can be divided by sub-specialty. ICU, IMCU, PACU, and ER are considered critical care areas in most facilities.
If your patient "goes bad", in other words, becomes more ill, vitals become unstable, they need any aggressive airway or blood pressure management, they go to the ICU. The inverse applies. When a patient stabilizes and is well on the road to recovery they transfer to the floor for management.