Volume Rendering in simple words I am looking for a explanation about volume rendering in simple words, a step by step kind of, like ray tracing: 


  for every pixel in the screen plane, trace a ray starting from the eye
  point to the screen pixel location, a compute the nearest objet
  intersection in the scene, calculate the pixel color, and so on.


something like that
Volume rendering is not like ray-tracing, it is like "scene rendering". i.e. there exists several algorithms to render volumes.

One close to ray-tracing is ray-marching, and has may variants. The simplest:
for every pixel in the screen plane, trace a ray starting from the eye point to the screen pixel location, and advance along the ray by constant steps. At each voxel calculate the pixel color and transparency, blend it to the pixel value, and so on.

Blend = 


$C_{pix} += T_{pix}*C_{vox}*(1-T_{vox})$ 
$T_{pix} *= T_{vox}$


with $C_{vox},T_{vox}$ the color and transparency of the current voxel, and $C_{pix},T_{pix}$ the cumulated color and transparency of the pixel.