A domain name represents your "address" on the Internet. It is a phrase containing several components separated by a period ("dot").
Each domain name includes a "Top level" domain such as .COM, .NET, or .ORG which originally specified the broad category of an organization. Thus, domain names ending in .COM were intended for commercial, for-profit organizations, while names ending in .NET were for network infrastructure machines, and those ending in .ORG were used for miscellaneous, usually non-profit organizations. Today these connotations still exist, although there is no actual requirement to follow the categories. Anyone is free to register their name in either or all of the .COM, .NET, or .ORG Top-levels.
"Second level" domains represent your specific identity, as in YOURNAME.com
In an e-mail address, the domain name follows the @ symbol, so that John or Jane Smith's address at this company might be: jsmith@yourname.com
Internet computers that track and locate each domain name actually use numeric addresses (eg. 198.197.196.195) , so that each domain name is assigned a numeric equivalent. You don't have to worry about this since Domain Name Servers (DNS) take care of the matching. However, this is the reason why functioning DNS is required before domain names can be registered.