it is used to raise an application.

It is used for raising the error from sql statement 
Eg shown below

  IF @@TRANCOUNT>0 ROLLBACK TRANSACTION    
                   Raise an error with the details of the
exception    
                  DECLARE @ErrorMessage NVARCHAR    
                  (    
                          MAX    
                          )    
                          DECLARE @ErrorSeverity INT;    
                  DECLARE    
                          @ErrorState INT;    
                          SELECT    
                          @ErrorMessage=ERROR_MESSAGE    
                          (    
                          )    
                          ,    
                          @ErrorSeverity=ERROR_SEVERITY    
                          (    
                          )    
                          ,    
                          @ErrorState=ERROR_STATE    
                          (    
           )    
                          ;    
                          -- Use RAISERROR inside the CATCH
block to return error    
                          -- information about the original
error that caused    
                          -- execution to jump to the CATCH
block.    
                          RAISERROR    
                          (    
                                  @ErrorMessage,-- Message
text.    
                                  @ErrorSeverity,--
Severity.    
                                  @ErrorState -- State.    
                          )    
                          ;

Raise Event is used for raising events. ex raising control 
event from main page .

The RaiseEvent statement raises an event previously declared
with the "Event" statement. Since only classes can raise
events, usage of this keyword is reserved for class modules,
forms and user controls.