DIP and PLUNGE are both measured from the horizontal. However, remember that the TREND/PLUNGE convention, when used for representing planes, represents the orientation of the POLE VECTOR of a plane, which is of course normal to the plane. Hence, for a given pole on a stereonet, the relationship between the TREND/PLUNGE and DIP/DIPDIR coordinates, is:
DIPDIR = TREND + 180 (or vice versa)
DIP = 90  PLUNGE (or vice versa)
Remember:

Poles on a pole plot are projections of the pole vectors of planes.

DIP/DIPDIR is a "plane vector convention".

TREND/PLUNGE is a "pole vector convention", and is used in Dips to represent the orientation of linear features (i.e. lines in 3dimensional space), or the pole vectors of planes.
See the Convention topic for more information.