I don't know how to understand the class "Application" ,can anybody explain for me ,give a example ,thanks!
It's just a class full of static helper functions/properties to do with the application as a whole, e.g. level loading, finding the path to the data folder, etc.
e.g.
Application.LoadLevel("name"); //loads a level called name
Debug.Log(Application.dataPath); //prints out the path to the data folder
Application.Quit(); //quits the application (except when in editor mode)