A domain is basically a main web address of a site for example www.youtube.com. That is the main URL that a company has and through this domain the customer can go through the website using the domain. Ranking system is also there according to reference of the search.
A domain name takes the form of two main elements.
The primary one is the name of the company and the later it has . com for example youtube is the primary name and .com is secondary which is common for all.