When working with lots of windows open, you can easily get lost as one slides behind another etc etc.
Ghoster is designed to ensure that only those windows that are important are prominent on your desktop. Ghoster is a small program which hides all those windows that aren't important and thus allows you to more easily see what's active and what's not.
Although Ghoster is highly configurable the downside is that the configuration of Ghost can only be changed by text file. Therefore, if you don't have any coding experience, you won't be able to change it one bit. Because of this, it's not really suitable for non Mac experts although if you're happy to leave it in it's default state, it's OK.
Overall, Windows itself generally does a much better job of helping you manage open windows and applications than Ghoster but if you find yourself really in a constant mess, you might find some use for it.