Domain is your web address for which all can access your website. At first, you prefer a domain name like www.your preferable name.com or .in or .co.in or .net or .biz or .edu or .org, etc.
Then you search for your preferable name into searching site like whois, godaddy, etc. If you get your preferable name for domain it will be good but if do not get your preferable name at first your preferable name must be customised until you get your suitable domain name.
You will give the charges for domain for one year first. After this year renewal will be must otherwise your domain will be cancelled.
Domain name is not only a getway of your website but also a recognition of your organisation. So please be careful to choose the suitable domain name in time of domain registration.