Now for the obligitory ----- Google is your best friend!
http://www.mozilla.org/ will answer your questions.
Mozilla is an open source (free) web browser. In the Mozilla family you will find Firefox, Pheonix, Chatzilla, and other products.
Alot of the members here on 'AO' use a different browser than IE. Some use the Mozilla products and some use Opera, and others. I use the Mozilla Firfox browser myself.
Welcome to 'AO'.