Top Level Domains
A top-level domain (TLD) is one of the domains at the highest level in the hierarchical Domain Name System of the Internet. It is the part of the domain located to the right of the dot, e.g. denic.de.
A general distinction is made between generic Top Level Domains (gTLDs) such as .com or .info, and country code Top Level Domains (ccTLDs) such as .de.
Responsibility for the country code Top Level Domains (ccTLDs) is in the hands of the local Internet Community in each country. For this reason, the conditions governing the registration of domains may vary from one country to another. In Germany, there are few restrictions, and these are all stated clearly in DENIC's Domain Guidelines. For example, the number of domains that may be registered by an individual or a business is not limited. Due to this liberal registration policy and the large number of providers on the market administering domains for customers, .de is amongst the world's biggest TLDs: Measured by the number of domains registered under .de, the German TLD ranks number two worldwide of all ccTLDs with a clear geographic focus. Still most domains are registered under .com.
You find an overview of the approximately 240 currently existing country code Top Level Domains on the website of IANA, which also lists the names and addresses of the registries in charge of each of them.
By contrast to the ccTLDs, the generic TLDs do not necessarily have a geographical focus. To give a few examples: .biz indicates commercial offers by businesses, .org stands for "organizations", and under .gov you will find the Internet pages of the US government, whereas .int is reserved for international organizations such as the United Nations. For information about the gTLDs and the new gTLD process, e.g. for regional TLDs, City TLDs and TLDs such as .sport or .bank please refer to the website of an international organization called ICANN, which is responsible for coordination of these TLDs. For a few years now, there have also been some TLDs which are no longer in line with this strict distinction, e. g. the geographical TLDs .eu or .asia.
Domain Information via Whois
Most registries make information about the domain publicly available, as well as the contact data of the domain holder and that of their contact persons for administrative and technical purposes. This ensures that the domain holder can be found in case his/her domain infringes the rights of third parties, or that the technical contact person may be contacted in case of technical problems. This service is usually referred to as "whois". In many cases, the whois service of a registry may be accessed directly through a website. Registries which make all data of a domain directly available are called "thick registries". This means that the data of the domain holder, the various contacts and the technical information are collected and maintained centrally by the registry. DENIC is an example of this type of registry. By contrast, a so-called "thin registry" such as Verisign, which administers the TLDs .com and .net, will refer the enquirer to the page of the individual registrar who has handled the registration in question. This is due to the fact that in case of a "thin registry", information about domain holders and other contact data is not kept centrally, but by individual registrars.