Viz (or Visibility) is how far you can see a static, unlit object.
Runway Visual Range (RVR) is how much of the runway you can see and takes account of the Runway lighting. This is how you can get a reported Visibility of 150m but the RVR might be 300m or more.
In the old days RVR used to be calculated by having someone stand on/near the runway and count how many runway edge (or centreline) lights they could see - ATC would then multiply that number by the distance between the lights and, hey presto, that was your RVR. Nowadays there are electronic systems to work out the range by shining a light at a receiver and then it works out the equivalent RVR. This is called an IRVR.
As you will be aware each type of approach has it's minimums. For a non-precision (i.e. visual) approach you need the Visibility to be above minimums. For a precision (i.e. vectors to ILS) approach you need the RVR to be above minimums.