Oh yeah, there's a hierarchy alright...
HA's, or Healthcare Assistants : They are who we informally call
'ah mah's. They do the general legwork at times, like fetching
medicine from the pharmacies, helping to serve the meals, washing
the cups, etc. Some of their legwork is now being taken over by
Porters.
PCA's, or Patient Care Assts : They help patients with tasks like
showering, sponging, feeding, etc and tasks are delegated to them
by the Enrolled Nurses or the Staff Nurses.
Enrolled Nurses : They used to be called ''Asst Nurses''. They play
an invaluable role as the better and best EN's really make a huge
difference between a smooth, uneventful shift, or one that's
inefficient. EN's are increasingly becoming independant with their
own roles and responsibilities.
Staff Nurses Grades I & II : These are the nurses who are being
assigned patients to take care of. They are the liaison between the
doctors, pharmacists, allied healthcare professionals (eg.
Dietitian, Physiotherapists, Occupational Therapists, etc),
patients and their relatives. Nurses are responsible for ensuring
that patient's needs and all medicine administration and any
ordered medical procedures are completed during their shift, and
they write reports to pass on to the oncoming shift. They ensure a
smooth flow of tasks for the patient. In resuscitation efforts,
nurses work closely with the doctors. Documentation and PR are
integral parts of a Staff Nurses' job.
Snr Staff Nurses Grades I & II : They are responsible for the
running of certain ward areas, as well as the delegation of tasks
to all under them, and the training and guidance of their juniors.
Huge responsibility! Snr Staff Nurses are equipped with experience
and knowledge and the best amongst them can actually suggest better
ways and methods of accomplishing certain tasks to the Housemen and
even the Medical Officers. 
Beyond that, you have those Nurse Clinicians, Nurse Specialists,
Nurse Educators, and Nurse Managers, all called 'Sisters' because
they're at management level.
On top of those, you have the Matrons aka the Asst Directors and
Directors of Nursing Administration. They sit in an office and no
longer participate in clinical work but instead, run the hospital
and take care of all the nurses employed there.
Hope this helps!
