Thumb This is a 100% Opaque element, everything inside this tag is fully visible

Thumb The background of this section has a 60% opaque background, set with rgba color.

 

The image, text and border remain 100% visible.

Thumb This section has a CSS3 gradient background using rgba colors from 90% to 10% transparency.
Thumb And of course you can combine opacity, transitions and rgba background colors to get awesome effects.