A domain name is the address of your website that people type into a browser URL to visit your website. Domain names exist because to actually find the location of your website would mean you would have to memorise a number something like 192.168.1.1 for every single website you visit or need to visit.
When you enter a domain name in your web browser, it first sends a request to the DNS server. The DNS sever then does a lookup for the name servers associated with the domain and forwards the requests to the name servers. The name server then forward the request to the web server, and send the data from the web server back to the browser.
There are different types of domain names with many different extensions. The most popular extension is .com but there are others such as .net .org .info
A top level domains are generic domain extensions that are listed at the highest level in the domain name system. TLDs are .com or .org
County code top-level domain are country specific domain names which end with a county code such as .uk for the United Kingdom.
Sponsored top level domain is a category of TLDs that has a sponsor representing a specific community served by the domain extension. For example .edu for education related organisations, .gov for government sites.
The Internet Corporation for Assigned Names and Number (ICANN) manages the domain names system. It is a non-profit organisation that creates and implements the policies for domain names. ICANN gives permission to companies called domain name registrars for selling domain names. The domain registrars are allowed to make changes to the domain names registry on your behalf. The domain name owner is responsible for telling the registrar where to send requests and for renewing their domain registration.
A subdomain is a child domain under the main name, for example if you wanted a store for the website www.silvertoad.co.uk it may be listed with the sub domain store.silvertoad.co.uk. When you register your domain you have permission to create subdomains for it yourself.