I am thinkin of adding a sub-domain to my existing domain as showcase.domainname.com. I just wanted to know what is the difference between showcase.domainname.com and domainname.com/showcase
It's the same thing if I'm not wrong, but you gotta do a configuration in your cpnael or whatever, so your domainname.com/showcase will be the same thing as your showcase.domainname.com .
If you refer which one is better in term of SEO, this is a good read for you : http://www.mattcutts.com/blog/subdomains-and-subdirectories/ - Dynashox -