A conventional pillow top is sewn onto the top surface of the mattress, with a gap between the pillow top and the mattress itself. A Euro pillow top (also called a Euro top), by contrast, has the padding underneath the top cover and is flush with the mattress edges, so it doesn't look like an extra layer sitting on top of the mattress.
In addition to creating a cleaner look, the other major benefit of a Euro pillow top is that, because it is stitched underneath the cover, it won't shift or lose shape the way a conventional pillow top can. Euro top fillings also tend to be denser than their standard counterparts, making them somewhat more durable as well.