Such as the saracasm tag (</sarcasm>), the </pwned> tag represents, that some one has been pwned in a livechat or internet forums. The "</" and ">" come from html talk used on .html, pages for internet websites.
"Haphazardly: Hey man you suck! And your girlfriend was a ho!, </pwned>"
"Nitrous69: I wish I was cool as Haphazardly!"