Here is an extract from the explanation provided by Internic, whose copyright we acknowledge.

The Domain Name System: A Non-Technical Explanation – Why Universal Resolvability Is Important

What is the Domain Name System?

The Domain Name System (DNS) helps users to find their way around the Internet. Every computer on the Internet has a unique address – just like a telephone number – which is a rather complicated string of numbers. It is called its "IP address" (IP stands for "Internet Protocol").

But it is hard to remember everyone's IP address. The DNS makes it easier by allowing a familiar string of letters (the "domain name") to be used instead of the arcane IP address. So instead of typing 192.0.34.65, you can type www.icann.org. It is a "mnemonic" device that makes addresses easier to remember.

Translating the name into the IP address is called "resolving the domain name." The goal of the DNS is for any Internet user any place in the world to reach a specific website IP address by entering its domain name. Domain names are also used for reaching e-mail addresses and for other Internet applications.

What is universal resolvability and why is it important to users?

Think of the phone system . . . when you dial a number, it rings at a particular location because there is a central numbering plan that ensures that each telephone number is unique. The DNS works in a similar way. If telephone numbers or domain names were not globally unique, phone calls or e-mail intended for one person might go to someone else with the same number or domain name. Without uniqueness, both systems would be unpredictable and therefore unreliable.

Ensuring predictable results from any place on the Internet is called "universal resolvability." It is a critical design feature of the DNS, one that makes the Internet the helpful, global resource that it is today. Without it, the same domain name might map to different Internet locations under different circumstances, which would only cause confusion.

When you send an e-mail to your Aunt Sally, do you care who receives it?

Do you care if it goes to your Uncle Juan instead? Wait a minute…do you have an Uncle Juan? Then whose Uncle Juan received it? Do you care if it reaches Aunt Sally if you send it from work but my Uncle Juan if you send it from home?

Of course you care who receives it . . . that's why you wrote it in the first place. Whether you're doing business or sending personal correspondence, you want to be certain that your message gets to the intended addressee.

If at any point the DNS must make a choice between two identical domain names with different IP addresses, the DNS would not function. It would not know how to resolve the domain name. When a DNS computer queries another computer and asks, "are you the intended recipient of this message?", "yes" and "no" are acceptable answers, but "maybe" is not.

Where does ICANN come in?
This is where ICANN comes in . . . ICANN is responsible for managing and coordinating the DNS to ensure universal resolvability.

ICANN is the global, non-profit, private-sector coordinating body acting in the public interest. ICANN ensures that the DNS continues to function effectively – by overseeing the distribution of unique numeric IP addresses and domain names. Among its other responsibilities, ICANN oversees the processes and systems that ensure that each domain name maps to the correct IP address.

What goes on behind the scenes?

Behind the scenes, the story becomes a little more complicated.

In an Internet address – such as icann.org – the .org part is known as a Top Level Domain, or TLD. So-called "TLD registry" organizations house online databases that contain information about the domain names in that TLD. The .org registry database, for example, contains the Internet whereabouts – or IP address – of icann.org. So in trying to find the Internet address of icann.org your computer must first find the .org registry database. How is this done?

At the heart of the DNS are 13 special computers, called root servers. They are coordinated by ICANN and are distributed around the world. All 13 contain the same vital information – this is to spread the workload and back each other up.

Why are these root servers so important? The root servers contain the IP addresses of all the TLD registries – both the global registries such as .com, .org, etc. and the 244 country-specific registries such as .fr (France), .cn (China), etc. This is critical information. If the information is not 100% correct or if it is ambiguous, it might not be possible to locate a key registry on the Internet. In DNS parlance, the information must be unique and authentic. Let us look at how this information is used.

Scattered across the Internet are thousands of computers – called "Domain Name Resolvers" or just plain "resolvers" - that routinely cache the information they receive from queries to the root servers. These resolvers are located strategically with Internet Service Providers (ISPs) or institutional networks. They are used to respond to a user's request to resolve a domain name – that is, to find the corresponding IP address.

So what happens to a user's request to reach our familiar friend at icann.org? The request is forwarded to a local resolver. The resolver splits the request into its component parts. It knows where to find the .org registry – remember, it had copied that information from a root server beforehand – so it forwards the request over to the .org registry to find the IP address of icann.org. This answer is forwarded back to the user's computer. And we're done. It's that simple! The domain name icann.org has been "resolved"!

Why do we need the resolvers? Why not use the root servers directly? After all, they contain essentially the same information. The answer is for reasons of performance. The root servers could not handle hundreds of billions of requests a day! It would slow users down.

If you are still with the story, you are already wondering about more complicated names with more parts such as www.icann.org. Well, the DNS is a hierarchical system. First, the resolver finds the IP address for the .org registry, queries that registry to find the IP address for icann.org, then queries a local computer at that address to find the final IP address for www.icann.org. Just what you would expect.

It is important to remember the central and critical role played by the root servers that store information about the unique, authoritative root. Confusion would result if there were two TLDs with the same name: which one did the user intend? The beauty of the Internet architecture is that it ensures there is a unique, authoritative root, so that there is no chance of ambiguity.
What about "alternate roots?" How do they fit into this picture?

Anyone can create a root system similar to the unique authoritative root managed by ICANN. Many people and entities have. Some of these are purely private (inside a single corporation, for example) and are insulated from having any effect on the DNS. Some, however, overlap the authoritative global DNS root by incorporating the unique, authoritative root information, and then adding new pseudo-TLDs that have not resulted from the consensus-driven process by which official new TLDs are created through ICANN. The alternate root operators persuade some users to have their resolvers "point" to their alternate root instead of the authoritative root. Others (New.net is a recent example) also create browser plug-ins and other software workarounds to accomplish similar effects. The one uniform fact about all these efforts is that these pseudo-TLDs are not included in the authoritative root managed by ICANN and, thus, are not resolvable by the vast majority of Internet users.

Why do alternate roots create a problem?

There are many potential problems caused by these unofficial, alternate root efforts to exploit the stability and reach of the authoritative root. These efforts are often promoted by those unwilling to abide by the consensus policies established by the Internet community, policies designed to ensure the continued stability and utility of the DNS.

For example:

First, the names of some of these pseudo-TLDs could overlap TLD names in the authoritative root or those that appear in other alternate roots. Our familiar friend icann.org could appear in two different roots. Your e-mail to Aunt Sally could end up with my Uncle Juan.
Second, the unknowing users might not be linked to one of these alternate roots and not be able to reach these pseudo-TLD addresses at all. Your e-mail to Aunt Sally could end up as a dead-letter.
Third, those purchasing domain names in these pseudo-TLDs may not be aware of these and other consequences of the lack of universal resolvability. Or they may be under the impression that they are experiencing universal resolvability when in fact they are not. They may be very upset to learn that the names they registered are also being used by others, or that a new TLD in the authoritative root will not include those names.
These problems are not significant so long as these alternate roots remain very small, that is, house few domain names with little potential for conflict. But if they should ever attract many users, the problems would become much more serious, and could affect the stability and reliability of the DNS itself. Users would lose confidence in the utility of the Internet.

What is ICANN's role?

ICANN's mission is to protect and preserve the stability, integrity and utility – on behalf of the global Internet community – of the DNS and the authoritative root ICANN was established to manage. ICANN has no role to play with alternate roots so long as these and other analogous efforts do not create instabilities in the DNS or otherwise impair the stability of the authoritative root. But ICANN does have a role to play in educating and informing about threats to the Internet's reliability and stability.

ICANN is a consensus development body for the global Internet community, and its focus is the development of consensus policies relating to the single authoritative root and the DNS. These policies include those that allow the orderly introduction of new TLDs.

There are those–including operators of commercialized alternate roots–who pursue unilateral actions outside the ICANN consensus-development process. Many hope to circumvent these processes by claiming to establish some prior right to a top-level domain name. ICANN, however, recognizes no such prior claim. ICANN will continue to reflect the public policy consensus of the global Internet community over the private claims of the few who try to bypass this consensus.

In Short . . . . . .

Just as there is a single root for telephone numbers internationally, there must be a single authoritative root for the Internet, administered in the public interest.

The makeup of domain names
The letters after the dot signify the top level domain (“TLD”) for a name. TLDs were originally limited to .com for business, .net for Internet business, .ac for academia, .gov for government, and a small number of others. It soon became apparent that each country needed its own individual identity. Now every country in the world has its own TLD.

All TLDs are allocated, approved, and registered by ICANN. More recently ICANN has authorised many more names which are not country specific, such as “.biz”, “.info”, “.name”, “.aero”.

Most countries have their own central registrar for their particular TLD. In some cases the registrar is privately owned. In most it is a governmental organisation. The rules as to who and how a new domain name must be registered vary greatly from one country to another. In most cases a number of sub registrars are appointed. These are private “for profit” organisations who simply act as wholesalers to the general public. These arrangements are such that it is very easy to register a “.com” or a “.co.uk” for your UK business. It is far more difficult to register a “.fr”.

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