Design Patterns - WWW (What/ When / Why)

• What is a design pattern? - Someone has already solved your problem - (Head First Design Pattern).
• When should we use it? - Depend upon what kind of problem you want to solve .. there might be already a mature solution (in design) for the problem.. so that means you need to spend some time understanding existing design patterns and where you should apply them.. if you think none of the existing design pattern suits to your problem then you can share your experience with the community and you never know .. might be the next design pattern belongs to you..
  
• Why should we use it? As already said - Someone has already solved your problem.. and when you apply it your code it make self explanatory . so that if some one else is looking to your code can easily make what kind of problem you are trying to solve.. ah you got free documentation...