Duckworth-Lewis(D/L) method in cricket

We all have heard about "Duckworth-Lewis (D/L) method" but many don't know how they come up with figures for the match. Many a times, we see the score on board arrived by using the said method is more than our calculation. I would like our member to put more light into the topic with examples.
My query are:
1. How is the DLS calculated in a given match?
2. When and how can DLS method be applied?
3. What is the basic concept of this method?
4. Is this method apt for the match or situation?
5. Do this method give justice to both the team?