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Translucency is basically the same thing as the surface attribute of the same name, but done up as a node, which lets it be controlled in more ways, including a separate translucency color... One nice thing about the Translucency node is that it offers two modes or ranges. When set to 90 degrees, it works just like the familiar Translucency channel, where the surface is most brightly lit when the surface is perpendicular to the light rays, and not lit at all when the surface is edge-on to the light. Basically, it's the same as the Lambert diffuse shading, but on the sides of the object that face away from the light. On a sphere, with both diffuse lighting and translucency, this results in a dark band along the terminator, where the surface isn't facing the light. Wit the 180 degree range, this band is gone. Now, the surface, when using only Translucency, will be brightest where it's facing away from the light, 50% as intense along the terminator, and 0% at the point that's facing directly toward the light. This node is best used with another shader since the shading it generates isn't all that useful on its own. It will actually tend to look more like a negative. When combined with Lambert, Oren-Nayar or Minnaert (usually with a Mixer or Add node), this shader does a relatively good job at translucent surfaces, like paper, leaves, lampshades, or anything else that's thin. The color and amounts can both be textured, so you can use this to fake underlying textures, like paper grain, or veins in the surface, that are only visible when backlit.