BlueStacks (whose full name is BlueStacks App Player), is what could be called a player of Android applications on a Windows PC which took the creators over two years to develop (and you can even run it on a Mac). That is, with BlueStacks you can run and use the apps on your Android smartphone or tablet in Windows: games, tools, messaging programs, including the famous Whatsapp.
BlueStacks is very useful, for example, to test an app on your PC before installing it on your Android device, as well as for viewing everything on the big screen which means doing things more comfortably, etc. When installing BlueStacks on your Windows PC, you are provided with some apps for you to try, but have the option to download others with the help of an Internet connection.
The fame and acceptance of BlueStacks as an Android applications emulator in Windows is increasing since its launch. However, you should keep in mind that BlueStacks doesn't support all Android applications. So, if Candy Crush saga is your favourite game, or you prefer Telegram or Temple Run 2, etc, and you want to play it on your PC, you will need BlueStacks, but don't worry because it's free to download and use.
NOTE: Before you can install BlueStacks on your Windows XP, you need to install Service Pack 3.