This cannot be explained in a few sentences, this would be a book.
So only some differences:
1. Internet Explorer comes along with Windows, Safari with Mac OS. All other browsers need to be downloaded on these systems.
2. Most browsers are based on a different rendering engine, what causes different speeds and sometimes different behaviour. Earlier IE's were pretty bad in layout.(Boxmodel) An actual and easy to find difference is alignment. In HTML u can align text, images etc. left, right, centered or just omit alignment. If you omit alignment, every browser will use "left", except Opera. All tags with no alignment will be centered there.
3. At least, there is the usability. Many peoble prefere FireFox, because it's easy to use and there are lots of useful plugins, like AdBlock, NoScript or Download them all(my favorites
). I prefere FireFox, because i'm using Windows and Mac OSX and the look and feel of FireFox is on both systems nearly the same. Other people prefer Opera, because it has many useful functions included like SpeedDial or TrashCan. And then there are users who just take the browser, they are used to.
