A subdomain is a domain that is part of the main domain.
asubdomain.example.com is a subdomain of
www.example.com is also a subdomain of
example.com - there is nothing special about the characters
www other than that they are frequently used by companies for their front facing website.
Whether to use a subdomain or subfolder is a question that gets debated a lot, many SEOs would say there is no right answer - and many have a preference for one or the other depending on the situation.
If you want to have a more separate website, isolated from your main domain, a subdomain could be a good choice. Whereas a directory will share authority with your main domain.
Content last checked for accuracy and updated: 23rd November 2020, by Colin McDermott