.us stands for "United States". It is a country in North America. It may not be very well known, maybe quite small and not very influential. Maybe that explains why you haven't heard from it. I think it is a small country nearby much better-known countries such as Belize, Bahamas and Mexico :hehe:
As for buying .US : officially you need to live in the US or be a US citizen living abroad. Or be a business that can prove to have business affiliation with the US. I believe though some registrars will provide an American address for foreign customers (they may charge an extra cost for you using the US address of the registrars), this way you can own a .US address even as a foreigner. I am not sure which registrars do this though, I never tried getting one because the extention doesn't appeal to where I live (Europe).
I think .us is not interesting at all to non-Americans, just like .eu is only interesting to Europeans.
.US is and remains a country domain, even when the country is big and famous then it still remains of no use to the average citizen of Luxembourg, Lesotho, Laos, Uruguay, ...