Internet Explorer is a part of your Microsoft Windows operating system. It is not a freeware, shareware, or purchased program.
It is the default browser on your Windows box.
However, some companies (like AOL, for instance) will "brand" the browser; Meaning they've replaced some graphics and the line at the top of the window.
It's still Internet Explorer.
So when you say, "why are people using extra browsers like internet explorer?" you're way off the mark; Because internet explorer is the default, and most widely used browser on the planet.
We use other ones because
a) it sucks
b) it's entangled in too may other microsoft services; Outlook, MSN, etc.
c) it doesn't have some much needed features
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I use firefox because I'm running Linux.