Perhaps you have stumbled on subdomains while doing online searches. If you are curious as to why anyone would use them in the first place and if you should feature them on your site, then it is important to understand their purpose and of course the different benefits. For those who are not so sure what a subdomain is, well, think of it as an extension of an original top-level domain, or rather a third-level domain, so “Signup.yourwebsitename.com” versus the original, top-level site “yourwebsitename.com.” Ultimately, you will have a better idea of how subdomains affect SEO.