or what we use after @ in email address. like firstname.lastname@example.org here gmail.com is domain name.
But technically there is an IP (Internet Protocol) address of server where your website is located. It is like telephone number of your friend in your mobile with nick name. For example in your mobile, you have saved friend name is "Tom" with mobile number 9199xxxxxxxx. For making call you type "Tom"
and call him but mobile takes Tom's number and makes call. In same way, whenever we type website name in address bar of browser, our computer takes ip address of website and makes connection.
Domain name is made up of parts. These parts are separated by dot(.) like images.google.com. Technically domain name is read from right to left i.e. com » google » images. "com" is top-level domain, "google" second-level and actual name of website. "images" is third-level domain or this is also called sub domain.The top-level names are called the generic Top-level Domains (gTLDs). TLD can be three characters or more in length. Here are some examples of TLDs
These names are registered with and maintained by the Internet Corporation for Assigned Names and Numbers (ICANN).
There are also top-level names with ISO 3166 international 2 characters country code like .in, .uk, .us, .au. These are called geographical domains.
Many countries also maintain own second level domains like co.uk, co.in, ac.uk.