What is cross browser compatibility?

Simply put, it is the way a website's layout will look different depending on which web browser is being used to view it. Certain functionality of active content may also be affected as well. Writing clean code will is always the best way to avoid cross browser anomalies.

The most common web browser is the Microsoft Internet Explorer (IE) but more and more when I check my server log files, I see a good percentage of users using Firefox. Firefox is my personal default web browser as I find it faster and more feature rich, but I always check to see what a new website will look and behave like through several browsers to be sure.

There are a plethora of web browsers available these days, Konqueror, Mozilla, Opera, Safari, Epiphany.... The wikipedia has an excellent comparison article here

As a responsible web developer, you should know what your site looks like thru other browsers you might be surprised!