What is the exact meaning of CAVOK and CAVU? How are they used by pilots and ATC?
CAVOK is formally defined in ICAO Annex 3 ( APP3-2 2.2 Use of CAVOK ) . You can hear it at the airports where there is no ATIS , when ATC communicates the weather info.
CAVU is not formally defined in Annex 3 .

When the following conditions occur simultaneously at the time of observation:
 a) visibility, 10 km or more, and the lowest visibility is not reported;
 b) no cloud of operational significance;
 c) no weather of significance to aviation as given in 4.4.2.3 and 4.4.2.6;

information on visibility, runway visual range, present weather and cloud amount, cloud type and height of cloud base shall be replaced in all meteorological reports by the term “CAVOK”.