I decided to do a "surface scan" on the question. (Google: "domain + registration + ICANN")

First thing that popped up was a news item from today, on a site called Red Herring:

VeriSign has agreed to settle a dispute with the Internet’s oversight agency that will allow the domain registrar and security company to retain its control over the .com and .net registry till 2012.

VeriSign sued the Internet Corporation for Assigned Names and Numbers in 2004, alleging the non-profit was interfering with the company’s ability to do business.

The settlement, announced late Monday, is also expected to increase prices of the .com domain lease by 7 percent and add at least $40 million in revenue for VeriSign in 2007, analysts said.

ICANN is an international organization that coordinates the Internet’s domain name system. It had appointed VeriSign as the “.com” and the “.net” registry operator in 2001.

Together, the two domains are estimated to comprise 90 percent of all the registered domain names in the United States and 53 percent of all the domain names in the world, according to ICANN.

[emphasis added]

From which I infer that all others comprise no more than 47% of the planet's total...

Next: this, from the FAQ page of InterNIC.net:

I've seen domain names ending with two-letter combinations, like .uk. What are the rules for registering in these domains?

Two letter domains, such as .uk, .de and .jp (for example), are called country code top level domains (ccTLDs) and correspond to a country, territory, or other geographic location. The rules and policies for registering domain names in the ccTLDs vary significantly and some are reserved for use by citizens of the corresponding country. You should check with the registrar offering ccTLD registration services regarding the specific terms and conditions for registration.

Some ICANN-accredited registrars provide registration services in the ccTLDs in addition to registering names in .aero, .biz, .com, .coop, .info, .museum, .name, .net, .org, and .pro, however, ICANN does not specifically accredit registrars to provide ccTLD registration services.

For more information regarding registering names in ccTLDs, including a complete database of designated ccTLDs and managers, please refer to http://www.iana.org/cctld/cctld.htm

From the same page of FAQs:

How much does a domain-name registration name cost?

Each registrar sets the price it charges for registering names, and prices vary significantly among different registrars. In addition, some registrars offer discounted or free registration services in connection with other offerings, such as web hosting. To see what offering best meets your needs, you should go to the web sites of several of the registrars listed in the Registrar Directory on this site.

And of course, ICANN has its own website.