Plastic (Transparent)

Diffuse

This can be thought of as the overall color of the material. Transparent materials will have little or no diffuse color applied.

Specular

This is the color and intensity of the reflection of light sources within the scene. Black will completely turn off reflections, while white will provide a very shiny plastic look. Realistic plastic does not have color in the specular value, so generally you should use some level of gray or white. However, adding color can give the plastic material a metallic effect.

Roughness

This will add microscopic levels of imperfections to the surface of the material when the values are increased. When set to 0, a material will appear perfectly smooth and polished. When the value is increased, the material will appear more rough as light is diffused across the surface.

Samples

Within the roughness parameter, you will find a samples setting. A low samples setting (8 or lower) will tend to make the surface look more noisy, which will give a more imperfect and rough look. As you increase the value, the noise will smooth out and provide a more evenly distributed roughness.

Diffuse Transmission

This will cause additional light to be scattered across the surface of the material which can simulate a translucent effect. It will increase render time, so if it is not needed, the recommendation is to leave it at black.

Specular Transmission

This can be thought of as the transparency of the material. Black will be 100% opaque and white will be 100% transparent. 
If a transparent glass or plastic is being created, the Diffuse Color should be black with all the color derived from this parameter. Transparent glasses or plastics should also have the specular set to white. If a cloudy plastic is desired, the Diffuse Color can be set to a very dark version of the color set here.