|The ones like .ru, .uk, and .tt are called ccTLDs, or Country Code Top-level Domains. General TLDs include .com, .net, .org, .info, and .biz. The term domain extension encompasses them all, and their use is widely varied. For example, the TLD .uk is for United Kingdom and replaced its predecessor .gb. It is a very restricted TLD, but the Second-level Domain .co.uk is much more available to the general public in and out of the United Kingdom. The ccTLD .sc used to be marketed for Scotland and South Carolina websites, but now must be used in connection with the Republic of Seychelles.|
Oddly enough, an accredited registrar in Russia is still doing registrations for .su, the now defunct Soviet Union.
Apologies if I don't respond to your reply immediately. I don't check this site daily.