A list of all valid domains is maintained by the ICANN and is updated from time to time. There is three types domain level.
1.Top level domains: The top level domains are the highest level of domain names of the internet like com, net, org.
2.second level domain: The second level domains are the co.uk. This are the often created based on name of the company, product of service etc.
3.Internationalized domain names: This domain name is based on ASCII and doesn't allow the representation of the name and words of many languages.