Glossary:
IE6
Internet Explorer 6 is a previous version of Microsoft's web browser software. This version was shipped with the Windows XP operating system and has been around since 2001.
The problems with IE6 and modern web sites are well documented on the web, with entire forums setup to deal with IE6 issues. These are generally considered to be due to poor CSS support and non-standard JavaScript support.
Even though this is an outdated browser there are still many users (around 25%) currently on the internet with IE6.