It is a directory containing domain names of TLD level, as: de, fr, uk, com, org. Every name is unique in a root. Each name is associated with one or more IP addresses.
The function of such a directory is to provide a user (usually a browser) with an IP address of a sub-directory containing the next part of the domain name.
The root is the first step in searching for the target site. Its contents is limited to TLDs and associated IP addresses.
According to ICANN, a self proclaimed monopoly, there is one and only one root, operated by Verisign, under contract with the US Department Of Commerce (DOC). Any modification of this root must be approved first by ICANN, then by DOC.
Actually there exists many roots created by other organizations, in order to allow users to access sites that, for various reasons, do not have a TLD in the ICANN root.
They are roots independent from ICANN, thus from DOC, created by private organizations, for users rejected by ICANN, or rejecting conditions imposed by ICANN.
First, brands or people who cannot pay the 6-digit fees required by ICANN for rental and maintenance of a Top Level Domain (TLD).
Second, web service operators or suppliers wishing to start their own business model in low connectivity areas (Africa, Asia, etc.). With their brand or product as TLD they can deliver a service to their local customers, leveraging lower fee connections.
A third example is users located in countries not registered formally as States by the UN, or whose ccTLD in their language is disputed or shelved by ICANN, in contradiction with the Tunis Agenda ( UN Summit on the internet in 2003 and 2005 after which an outcome document was approved by all Member States, and is the current base of internet governance).
[View WSIS site> www.itu.int/wsis/index-en.html]
A forth case using TLDs is the protection against the censorship of authoritarian governments, or seizure without trial by the US Department of Home Security (DHS), or just avoid the systematic monitoring and resale by Verisign of access to the ICANN root.
Note that no root offers hosting services (email, forum, archive, etc.), their role is limited to the conversion (resolution, in geek) of a TLD to an IP address.
In fact, they are not clearly identified as they use open roots for a corporate closed network (extranet) more secure than the basic internet. Customarily trading companies accept ICANN terms because they think there is no alternative and they are satisfied with a sub domain com, net, info, or otherwise. However Google (and Chrome) use their own root (copy of the ICANN root); its IP address is 18.104.22.168 or 22.214.171.124.
The situation has changed with the new TLD policy (announced by ICANN in 2008, implemented in 2012-2016). Any string could then be filed for a TLD (with some restrictions) and a steep fee ($ 185,000).
This new policy is far from welcome by trademark holders, who fear an explosion in disputed cases and malicious practices. Nevertheless these reservations were overridden by ICANN. Because of the high cost of renting a TLD, and a waiting line of several years, it is foreseeable that the open roots become attractive to commercial companies, in addition to or in substitution to the ICANN root.
E.g. Mont-Blanc, for yogurt, pen, and geographic area. Yes, it shall be possible, as long as they be registered in different roots.
Within the Icann framework, names such as: tube.com, tube.net, tube.org, etc. allow in principle to differentiate different domains under the same name.
Within the open root framework, if there are several .tube, one will distinguish them according to the root being activated.
When the root is set by a network manager of a company, an ISP or private network, enabling an open root requires no user intervention.
In a personal computer installation can be done by clicking on an installer or follow technical instructions found easily on Google (very simple on IPad, IPhone, a bit more cumbersome in Windows).
In the absence of user’s choice, the root activated by default on all PCs is the Icann’s one.
It is expected that future versions of browsers shall include the use of open roots and offer a root menu.
Either by one of its IP addresses, or by a nickname picked by the user, like in a cell phone directory.
As many as available IP addresses (v4 or v6) and, therefore, billions of them.