Is shapefile proprietary?

[marado]

"While the format is proprietary, the specification is open."

I found this website due to this sentence, and while I was hoping to learn more about it, there is no clarification at all to this sentence. What does it mean? I understand that the "specification is open" (and there's a link to it and everything), but what does "the format is proprietary" mean? What is the distinction between format and specification in this context?