When you are developing a website you want to see how it looks in different browsers. You can install a few browsers like Firefox, Opera, Chrome etc but you is not easy to test them on different operating system. You will need some operating system virtual images – for example for Windows XP, 7, 8, RedHat, MacOS etc. This is not an easy task.
You can get a lot of help from some online websites that will load your webpage in different browsers/operating systems.