When you are developing a website you want to see how it looks in different browsers. You can install few browsers like Firefox, Opera, Chrome etc but you it’s 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.