I don't want to get into the tomato - tomatoe thing here, but.... Transparency is attributed to images which cannot use the rgba. Opacity IS the reason for the Alpha call (the a in rgba). Your example is technically using opacity. And, yes it is used not only in straight colors but gradients, hovers, and other fun things. I was trying to keep it short of War and Peace.

(Note spacing /\ /\ /\ ) Without writing another book, a very good explanation/tutorial can be found at http://tinyurl.com/y929h5f6. I love the hover part even if it does probably muddy things by using "translucent." Knowledge is power.