RaiseError vs Throw

/** Raise Error**/
BEGIN TRY
DECLARE @result INT
SET @result = 1/0
END TRY
BEGIN CATCH
DECLARE
@ErMessage NVARCHAR(2048),
@ErSeverity INT,
@ErState INT
SELECT
@ErMessage = ERROR_MESSAGE(),
@ErSeverity = ERROR_SEVERITY(),
@ErState = ERROR_STATE()
RAISERROR ('error in number division',16,1);
RAISERROR (@ErMessage, @ErSeverity,@ErState);
RAISERROR (49903, 16, 1);  -- System related messages 
RAISERROR (51000, 16, 1); -- Not defined in system messages.
RAISERROR (51000, 25, 1) WITH LOG;; -- Change the @ErSeverity level from 16 to 25
PRINT 'End of error handling';
END CATCH

 SELECT * FROM sys.messages
 WHERe message_id=51000 
 --and severity=16
 SELECT * FROM  sys.syslanguages
 where lcid=1033


 --select min(message_id), max(message_id) from sys.messages
--HOW to Add custom error messages to sys.messages
EXEC sys.sp_addmessage @msgnum = 51000, @severity = 16, @msgtext  = N'This is a custom error';


BEGIN  TRY
 DECLARE @result INT
 SET @result = 1/0;
END TRY
BEGIN CATCH
DECLARE
@ErMessage NVARCHAR(2048),
@ErSeverity INT,
@ErState INT,
@Ernumber INT 
SELECT 
   @ErMessage = ERROR_MESSAGE(),
   @ErSeverity = ERROR_SEVERITY(),
   @ErState = ERROR_STATE(),
   @Ernumber=ERROR_Number();
--THROW; 
--THROW 50001,@ErMessage,1  - Here it will print the line 
--THROW @Ernumber,@ErMessage,@ErState -- Here you will get error,
--THROW 49903, 'RAM related message.', 1 -- System related error messages
THROW 51000, 'This is a custom error!', 1  -- Not defined in system messages
END CATCH
 


--NESTED throw statment
 
 BEGIN  TRY
 DECLARE @result INT
 SET @result = 1/0;
 END TRY
 BEGIN CATCH
 BEGIN TRY
 SELECT PARSE('2013JUNE32' AS DATETIME)
 END TRY
 BEGIN CATCH
 THROW;
 END CATCH;
  THROW;
 END CATCH 

 /*nested Raise statement*/

  BEGIN  TRY
 DECLARE @result INT
 SET @result = 1/0;
 END TRY
 BEGIN CATCH
 BEGIN TRY
 SELECT PARSE('2013JUNE32' AS DATETIME)
 END TRY
 BEGIN CATCH
RAISERROR ('format error', 16, 1);
 END CATCH 
RAISERROR ('divideby zero erro', 16, 1);
 END CATCH