Difference between ref and out keywords

What is the difference between out and ref in C#?
There is a big difference between ref and out parameters at the language level. Before you can pass a ref parameter, you must assign it to a value. This is for the purpose of definite assignment analysis. On the other hand, you don't need to assign an out parameter before passing it to a method. However, the out parameter in that method must assign the parameter before returning.
The following sample C# program uses both the ref and out parameter types,its enough for you to understand the actual difference between ref and out keywords:

using System;  

 class Program { 

  static void SetStringRef (ref string value)

   {  

   if (value == "cat") // Test parameter value 

   { 

    Console.WriteLine ("cat"); } value = "dog"; // Assign parameter to new value  

  }    

  static void SetStringOut (int number, out string value) 

   { 

    if (number == 1) // Check int parameter 

   { 

    value = "one"; // Assign out parameter 

   } 

  else 

  { 

  value = "other"; // Assign out parameter 

  }  

}  

static void Main() 

 {  

   string value1 = "cat"; // Assign string value 

   SetStringRef (ref value1); // Pass as reference parameter  

  Console.WriteLine (value1); // Write result    

  string value2; // Unassigned string 

  SetStringOut (1, out value2);// Pass as out parameter 

  Console.WriteLine (value2); // Write result  

}   
}

see out put:

ref and out usage examples

(program output)

cat dog one