ILS approaches fall into different categories (CAT I, CAT II, and CAT III) CAT III is further broken down to CAT IIIa, CAT IIIb and CAT IIIc.
Each have progressively lower landing minimums. CAT I ILS's generally require 1/2 mile visibility (either 2400 or in some cases 1800 RVR) RVR is runway visual range (the forward visibility, expressed in feet, a pilot can expect to see as measured by mechanical equiptment along side the runway) . CAT IIIc has the lowest visibility requirements, zero, the aircraft auto lands.
In order to fly any of the different category ILS's the airline, airport and flight crew must be certified.
Most airlines don't fly CAT IIIc approaches beacuse they realize the pilot still needs some forward visibility to taxi to the gate.
By the way, you'd probably get more replys to questions like this by posting them in the Tech/Ops section.