Definition of Domain name:
Domain name is an IP address which represents a physical point on the web. It is a part of the (URL) that usually expresses a domain name where to forward a request for a Web page. A domain name generally ends with an extension of 2 or 3 characters at all. These characters can indicate the country the website address is connected with or the sort of organization. A domain name discovers an organization or other object on the Internet. Here is the example of a domain name- www.itinfoworld.com

A Domain name easily locates an Internet address for “itinfoworld.com” at Internet point 104.219.248.78 and a specific host server named “www.” The “.com” part of the domain name redirects the purpose of the organization. This is called the top-level domain name. The “itinfoworld” part of the domain name describes the organization or object and along with the top-level is known as the second-level domain name. The second-level domain name usually records to and can be thought of as the “readable” version of the specific Internet address. Domain names generally operate under the Domain Name System (DNS), which is basically known as the address book of the Internet.

Know more details on Domain Name from here .

Domain name is an IP address which represents a physical point on the website. It is a part of the (URL) that usually expresses a domain name where to forward a request for a Web page. A domain name generally ends with an extension of 2 or 3 characters at all. These characters can indicate the country the website address is connected with or the sort of organization. A domain name discovers an organization or other object on the Internet. Here is the example of a domain name- itinfoworld.com

Now let’s we analysis this definition:

linkworld.us
let’s we analyse this domain as per my definition.
Here linkworld is a name
= A domain name generally ends with an extension of 2 or 3 characters.
Here this 2 characters is an  extension (.us)
= These characters can indicate the country the website address is connected with or the sort of organization
Here this 2 characters (US)  extension indicate the country (United States). 
So As per above definition  of domain name, linkworld.us is a domain name.

See more example:
* itinfoworld.org
Here, in this domain. this 3 characters (ORG) extension indicate the organization. that means, itinfoworld.org is an organisation.
* GlobalWebsHost.com
Here, in this domain. this 3 characters (COM) extension indicate that this is a commercial website. 

Which domain extension what indicates in a domain name?
Here are some popular domain extension and indication which you need to know for understanding the domain name and it will also help you to buy your perfect domain name.

.com – Commercial purpus
.org – Organization
.net – Network
.us – Unites States
.info – Information
.bd – Bangladesh
.in – India

Kinds of domain Name
Generally, there are 4 kinds of domain name.

1. Top Level Domain – resetcode.org
2. Second level Domain  –  Directly below a TLD in the DNS. Example:  .com.bd
3. Third level Domain – knowledgeworld.com.bd
4. Subdomain Name: help.answersmode.com

Read more details about kinds of domain Name.