I previously wrote this article in my blog, Think Big!.

The following example shows how query execution is deferred until the results is enumerated (requested in other words.)

static void TryLinq()
{    
   int i = 0;    
   int[] numbers = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };    
   // fake values for the query only (10 values)   
   var result = from n in numbers 
                select Increment(ref i);    
   Console.WriteLine("After query i = {0}", i); 
   // i still 0    
   Console.WriteLine();    
   Console.WriteLine("Enumerating results:");    
   foreach (var v in result)    
   {        
       Console.WriteLine("v = {0},ti = {1}", v, i);        
       // i is incremented every loop    
   }    
   Console.WriteLine("Press any key to continue . . .");    
   Console.ReadKey(true);    
   // Result:-    
   // After query i = 0    
   // Enumerating results:   
   // v = 1,  i = 1    
   // v = 2,  i = 2    
   // .............    
   // v = 9,  i = 9    
   // v = 10, i = 10    
   // What you get?    
   // Deferred-Execution / Lazy-Execution    
   // - Query doesn't execute until you    
   //   begin retrieving it's results.    
   // - Every time you try get a value    
   //   the query executes on this value only.
}
static int Increment(ref int i)
{
   return ++i;
}

Next is an example shows how you can immediate-execute the query.

static void TryLinq()
{    
   int i = 0;    
   int[] numbers = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };    
   // fake values for the query only (10 values)    
   var result = (from n in numbers 
                 select Increment(ref i)).ToList();    
   // The last call tries to get the value immediately.    
   Console.WriteLine("After query i = {0}", i); 
   // i is 10    
   Console.WriteLine();    
   Console.WriteLine("Enumerating results:");    
   foreach (var v in result)    
   {        
       Console.WriteLine("v = {0},ti = {1}", v, i);        
       // i still 10 every loop    
   }    
   Console.WriteLine("Press any key to continue . . .");    
   Console.ReadKey(true);    
   // Result:-    
   // After query i = 10        
   // Enumerating results:    
   // v = 1,  i = 10    
   // v = 2,  i = 10    
   // .............    
   // v = 9,  i = 10    
   // v = 10, i = 10    
   // What you get?    
   // Deferred-Execution / Lazy-Execution    
   // - Query doesn't execute until you    
   //   begin retrieving it's results.    
   // - Every time you try get a value the    
   //   query executes on this value only.    
   // - You can immediate-execute the query by    
   //   calling some conversation methods like    
   //   ToList or ToQuery.
}
static int Increment(ref int i)
{ 
   return ++i;
}

What you get?

Deferred-Execution / Lazy-Execution