A subdomain is a domain that is a part of a larger domain.
Since the Domain Name System is hierarchical, technically all domain are subdomains. The TLDs like .com are subdomains of the primary domain — the DNS system itself. Registered domains such as example.com are subdomains of the Top Level Domain represented in the domain extension (.com).
Normally, however, when we talk about “subdomains” we are talking about domains which are one more step below registered domains names, for example:
mail.example.com
blog.example.com
Should I use a subdomain?
That depends.
Subdomains can be very helpful for separating out different major aspects of domain usage. For example, some businesses like to have their main site at their primary domain (example.com), and then put their blog at a subdomain (blog.example.com).
Another common usage is for SaaS apps, especially when the public marketing site and the logged-in application are served from different computers or at least different code bases. Often the public site will be at the base domain, with the application on the subdomain (app.example.com).
The most common use of subdomains is when creating a network of sites which are individually manafged by other people, usually customers. This is the model for a number of popular blogging services, including WordPress.com and Tumblr.
