rollback transaction

http://msdn.microsoft.com/en-us/library/ms181299.aspx
In stored procedures, ROLLBACK TRANSACTION statements without a savepoint_name or transaction_name roll back all statements to the outermost BEGIN TRANSACTION. A ROLLBACK TRANSACTION statement in a stored procedure that causes @@TRANCOUNT to have a different value when the stored procedure completes than the @@TRANCOUNT value when the stored procedure was called produces an informational message. This message does not affect subsequent processing.

@@TRANCOUNT is incremented by one when entering a trigger, even when in autocommit mode. (The system treats a trigger as an implied nested transaction.)

example

CREATE TRIGGER [Purchasing].[dVendor] ON [Purchasing].[Vendor] 
INSTEAD OF DELETE NOT FOR REPLICATION AS 
BEGIN
  DECLARE @Count int;
  SET @Count = @@ROWCOUNT; 
  IF @Count = 0 
      RETURN;

  SET NOCOUNT ON;

  BEGIN TRY
      DECLARE @DeleteCount int;

      SELECT @DeleteCount = COUNT(*) FROM deleted;
      IF @DeleteCount > 0 
      BEGIN
        RAISERROR
           (N'Vendors cannot be deleted. They can only be marked as not active.', -- Message
                11, -- Severity.
                1); -- State.

        -- Rollback any active or uncommittable transactions
            IF @@TRANCOUNT > 0
            BEGIN
                ROLLBACK TRANSACTION;
            END
        END;
    END TRY
    BEGIN CATCH
        EXECUTE [dbo].[uspPrintError];

        -- Rollback any active or uncommittable transactions before
        -- inserting information in the ErrorLog
        IF @@TRANCOUNT > 0
        BEGIN
            ROLLBACK TRANSACTION;
        END

        EXECUTE [dbo].[uspLogError];
    END CATCH;
END;