Microsoft Internet Explorer is the default web browser on Windows. Thus various versions of Internet Explorer (usually referred to simply as IE) make up approximately 80-90% of all web browsing traffic. This results in much of the web being tailored to this application.
On Linux, there is no such default web browser. However, Mozilla and its various incarnations, especially Firefox, are popular, and KDE and GNOME emulate Explorer by having Konqueror and Nautilus, respectively, as default browsers.
Internet Explorer has also been widely criticized for various security holes and exploits. Its tight integration to the operating system has been claimed to only exacerbate the situation. In defense of anti-trust claims that Microsoft illegally "bundled" Internet Explorer with Windows, Microsoft countered that it is an integral part of the Windows operating system.
It is possible, but difficult, to run Internet Explorer in Linux via Wine. CodeWeavers CrossOver is a commercialized version of Wine that makes installing programs such as Internet Explorer much easier. The main reason for doing this is to be able to write standards-based web pages and test them in IE, as well as Linux-native browsers, to ensure that they render well in all browsers, without needing to dual-boot with Windows.