Direct volume rendering (DVR) can be described as a visualization of a discrete volumetric dataset
by mapping each value to a color and opacity. Each value in the dataset is referred to as a voxel,
which is a general representation for a value on a regular grid in three dimensional space. The
typically techniques used to render the dataset try to compute a discrete approximation of the
volume rendering integral.
Some additional general information about volume rendering can be found on this site.