You enter where you want to fly from, how much you want to spend, and when you want to go (either by season or month). You can also get more specific by entering how long of a flight you want, and even the types of activities you want. Then a map shows you the places you could visit within your criteria.
Then you can click on each of the prices for more details.
This is a really helpful tool, and can be very useful when trying to figure out where to go for your honeymoon.
What are your favorite travel sites?