[Brandon Martus]

Both PNG and GIF can have transparent areas.

PNG can actually be transparent, though, and have different levels of transparency. As in, your page has an image background, with a transparent PNG image over the top of it. However, IE does not support this well (read: at all), so keep that in mind.

So, you can overlay a PNG over an image, and see thru the PNG, but you cannot do that with GIF. Both PNG and GIF can have transparent areas (similar to how the rounded edges of the blue buttons on these forums are).