Short definition: dog crap
Long definition: The worst quality browser ever, this browser reeks of ass. Released on August 27th, 2001, it had little competition at first, and though it sucked, it (and other IE versions) gained a 95% market share around 2003. IE very nearly held a virtual monopoly by forcing it into every operating system, as well as the fact that in 2003 Netscape was long dead and Firefox had yet to be released, leaving just a few little known browsers. In 2004 Firefox rose up and started pissing on Internet Explorer 6's market share as people switched to the new, well engineered browser. By 2006 avid Internet users had switched to Firefox, and IE 6 was replaced by the slightly less crap Internet Explorer 7.
It was the most bug ridden Internet Explorer. Only a couple of years after IE7 superceded it, websites are dropping IE6 support. It had numerous security issues and had trouble displaying many web sites. One line of code can make it crash and anti-IE6 campaigns have been launched.
A pile of piss makes a better browser than Internet Explorer 6.