In the following image, there are two WHITE spheres sitting on a dark, reflective surface. Behind the two white spheres are a Red lamp (higher) and a Blue lamp (lower).

Click image for larger version.  Name: Translucency.jpg  Views: 271  Size: 19.1 KB  ID: 84675

Both spheres have Transparency, but only the right sphere has Translucency. The translucency setting controls how much light that falls on the object is able to pass through the object and illuminate the back side. Think of holding a flashlight to your hand in a dark room... or the way the Sun shines THROUGH a leaf and illuminates the back side even though it is technically in the shadow of the Sun.

For some things (like the above mentioned examples) or for example, light shining through a slightly dirtied window or light passing through a ruby, etc.; translucency will help mimic those objects in the virtual world.

Hope this helps!

James