I understand what ILS is, but what is the difference between that and CAT II and CAT III ILS?

the difference lies in the decision height minumum and the RVR (runway visual range). While ILS cat II provides a minimum decision height of not less than 100 feet abouve TDZ and a RVR not less than 1200 feet, ILS catIII provides an approach without a DH while the RVR of a catIII approach is split in 3 subcategories (catIIIa RVR=700feet, cat IIIb RVR=150 feet and cat IIIc RVR=0 feet).So if your arrival airport for example has reported a RVR of 200 you would fly an ILS cat IIIb approach.

Thanks for the reply