Browser compatability - a while back you could safely design your site for Explorer and forget about it. These days with all the security problems many people are switching to different browsers. The main browsers these folks are checking out are -
Mozilla - http://www.mozilla.org/
Firefox - http://www.mozilla.org/products/firefox/
Safari - http://www.apple.com/safari/ This is apples alternative.
The best thing to do is -
a) download the browser.
b) check out your site.
c) place an order.
How do things work - any glaring problems work on correcting them.