What exactly is the difference between opacity and transparency?
Opacity is the way you measure transparency in Illustrator. An object through which you can see other objects below it on the artboard is said to be transparent. How transparent is a question of opacity. Using the Transparency palette’s slider, you can assign an opacity of 0% to 100% to an object. In this respect, transparency and opacity are opposites; an object that has a 75% opacity setting is 25% transparent.