Every web site is viewed through a web browser. Most commonly Internet Explorer or Netscape, but there are many others. Here are some of the less common ones: Mosaic, Mozilla, Opera, Safari, and the rare: Amaya, Browse2000, Cyberdog, Cyber Passage, HotJava, NeoPlanet, OmniWeb... The list goes on!
Each browser reads web sites differently. They look and act differently and create many site builders serious headaches. Many developers build different versions of their sites to cater for each of the popular browsers increasing costs, development, maintenance and download time! This isn't necessary.
By adhering to web standards as defined by the W3C, you only need one version. You can use the W3C validator to test your site.