In short a domain or domain name is a string (e.g iregister.al) that allows people to easily navigate web servers and web pages.
Domain names are formed by the rules and procedures of the Domain Name System (DNS). Any name registered in the DNS is a domain name. Domain names are organized in subordinate levels (subdomains) of the DNS root domain, which is nameless. The first-level set of domain names are the top-level domains (TLDs), including the generic top-level domains (gTLDs), such as the prominent domains com, info, net, edu, and org, and the country code top-level domains (ccTLDs) like .al, .de, etc… Below these top-level domains in the DNS hierarchy are the second-level and third-level domain names (e.g .com.al, .net.al) that are typically open only for specific groups or purposes. All domain names are unique. This means there can be only one iregister.al in the world. You cannot register a name once it is registered by others.
The registration of these domain names is usually administered by domain name registrars who sell their services to the public.