The word Voxel is derived from the words volumetric and pixel. A voxel can be imagined as a pixel in threedimensional space. Such as a pixel being an element of a twodimensional image, a voxel is the element of a threedimensional image (or many slices of twodimensional images), representing a volume.
Voxels (so volume data) are needed mainly in medical applications (Computed tomography, Magnetic resonance imaging, ultrasound).
Voxels may be rendered using various Volume rendering techniques or by extracting iso-surfaces using Marching Cubes or Marching Tetrahedrons.