helllo                                         my name is mister burns
wot is CAT l , ll & lll? all i kno they are is runways and cat lll
are big runways with ILS stuff.'
But what actually are they???

CAT 1, CAT II and CAT III have different minimas and require different equipment in both aircrafts and ground stations. However they do have a thing in common: They are all ILS approaches

Where I fly, our runway is CAT 1, and that's the most basic ILS setup. That means a Decision height of atleast 200ft and a runway visible range of 550m.
And today we actually had some aircrafts forced into holding patterns since the RVR fell below 550m because of the fog  

To have aircrafts circle the airport because they cannot land is not good. That is why most international airports have upgraded to atleast CAT II. What that basicly means is that the minimas are lowered. CAT II is a DH between 200-100ft and an RVR of 300m.

The last ILS category is CAT III.
The CAT III, has 3 sub-categories:

IIIa= DH of 100ft or lower, but not less than 50ft. RVR of 200m

IIIb = DH less than 50ft, RVR of atleast 50m

IIIc = DH of 0, RVR 0m (landing completely blind)

CAT II and CAT III are different categories of precision ILS approaches, collectively referred to as low-visibilty procedures. CAT II and CAT III approaches are often terminated with the execution of an autoland. To be frank: You don't see much when the RVR is 300m  

Note: CAT IIIc has never been in commercial aviation, as it would require an automatic taxiing guidance system, however CAT IIIb is widely used.  

That's the Runway Visibilty Range
Here's the latest METAR for ESGP (my base)
ESGP 250450Z VRB03KT 0150 R01/0650N R19/0550N FZFG VV002 M00/M00  Q1019

According to it, there is a 150m visibility, but NOT at the runway thresholds which is 650m at Rwy 01 and 550m at Rwy 19.

Overall the weather today is bad: Fog (caused by advection) , calm winds (does a poor job of getting rid of the fog) and temp. very close to dewpoint.
I mean we don't even have a ceiling today (just a vertical visibility of 200ft), since the clouds and fog are seamlessy blended together  

Cat IIIb is FUN to fly. On the sim, there is no difference between any of them. The ILSs are all the same (ok so they may be named different, but they are all the exact same ILS equipment).