Sub-domain is linked with the main domain. Generally it uses in web 2.0 activity.

A subdomain is an area that is a piece of a bigger space; the main space that is not likewise a subdomain is the root area. For instance, west.example.com and east.example.com are subdomains of the example.com area, which thus is a subdomain of the com top-level space

Subdomains are just a small part of Internet hierarchy, but one with many advantages if you know when and how to use them.

It is a part of a larger domain.