Extensions are categories of Internet domain names. There are several of them to choose from:

.com represents the word "commercial," and is the most widely used extension in the world. Most businesses prefer a .com domain name because it is a highly recognized symbol for having a business presence on the Internet.
.net represents the word "network," and is most commonly used by Internet service providers, Web-hosting companies or other businesses that are directly involved in the infrastructure of the Internet. Additionally, some businesses choose domain names with a .net extension for their intranet Websites.
.org represents the word "organization," and is primarily used by non-profits groups or trade associations.
.biz is used for small business Web sites.
.info is for credible resource Web sites and signifies a "resource" web site. It's the most popular extension beyond .com, .net and .org.
.us is for American Web sites and is the newest extension. It has the largest amount of available names in inventory.
.biz extension. It is unrestricted and may be registered by anyone, from any country.
.tv is for rich content/multi-media Web sites, commonly used within the entertainment or media industry.
.ws was originally designated as the country code for Western Samoa, but is now commonly used as an acronym for "Web sites." It is unrestricted and may be registered by anyone, from any country.
.co.uk is the country code for United Kingdom. The .co portion of the extension indicates the domain name is for commercial purposes (similar to a standard .com extension). It is unrestricted and may be registered by anyone, from any country.
.org.uk is the country code for United Kingdom. The .org portion of the extension indicates the domain name is for an organization. It is unrestricted and may be registered by anyone, from any country.
.de is the country code for Germany. It is unrestricted and may be registered by anyone, from any country. If you are not a resident and citizen of Germany, the Administrative Contact for this domain name registration will automatically be assigned Network Solutions' German proxy contact information.
.eu is the country code for the European Union. Registration of .eu domain names requires registrants to meet one of the three eligibility requirements: have a registered office, central administration or principal place of business within the European Community; have an organization established within the European Community without prejudice to the application of national law; or be a natural person resident within the European Community. Alternatively, you can secure the use of a .eu domain name by using Network Solutions .EU Registration Service. Network Solutions Europe, LLC is the registrar for all .eu domain names. Click here for more information.
.be is the country code for Belgium. It is unrestricted and may be registered by anyone, from any country.
.cn is the country code for China. It may be registered by businesses, organizations and sole proprietorships from any country.
.com.cn is the country code for China. The .com portion of the extension indicates the domain is for commercial purposes (similar to a standard .com extension). It may be registered by businesses, organizations and sole proprietorships from any country.
.net.cn is the country code for China. The .net portion of the extension represents the word "network", and is generally used by internet infrastructure providers. It may be registered by businesses, organizations and sole proprietorships from any country.
.org.cn is the country code for China. The .org portion of the extension indicates the domain is for an organization. It may be registered by businesses, organizations and sole proprietorships from any country.
.cn Regional names. There are 34 regional .cn extensions available, for example, .bj.cn. The .bj portion of the extension represents Beijing and the .cn represents China. These extensions may be registered by businesses, organizations and sole proprietorships from any country.
.tw is the country code for Taiwan. It may be registered by businesses, organizations and sole proprietorships from any country.
.com.tw is the country code for Taiwan. The .com portion of the extension indicates the domain is for commercial purposes (similar to a standard .com extension). It may be registered by businesses, organizations and sole proprietorships from any country.
.org.tw is the country code for Taiwan. The .org portion of the extension indicates the domain is for an organization. It may be registered by businesses, organizations and sole proprietorships from any country.
.at is the country code for Austria. It is unrestricted and may be registered by anyone, from any country.
.com.mx is the country code for Mexico. The .com portion of the extension indicates the domain name is for commercial purposes (similar to a standard .com extension). It is unrestricted and may be registered by anyone, from any country.
.co.nz is the country code for New Zealand. The .co portion of the extension indicates the domain name is for commercial purposes (similar to a standard .com extension). It is unrestricted and may be registered by anyone, from any country.
.net.nz is the country code for New Zealand. The .net portion of the extension represents the word "network", and is generally used by internet infrastructure providers. It is unrestricted and may be registered by anyone, from any country.
.org.nz is the country code for New Zealand. The .org portion of the extension indicates the domain name is for an organization. It is unrestricted and may be registered by anyone, from any country.
.gs is the country code for the South Georgia & South Sandwich Islands. It is unrestricted and may be registered by anyone, from any country.
.tc is the country code for the Turks & Caicos Islands. It is unrestricted and may be registered by anyone, from any country.
.ms is the country code for Montserrat. It is unrestricted and may be registered by anyone, from any country.
.fm is the country code for Federated States of Micronesia, but is commonly used for FM radio.
.am is the country code for Armenia, but commonly used for AM radio.
.la is the country extension for Laos, but also used for Los Angeles, Louisiana or Latin America.