set XACT_ABORT ON - Rollback when transaction failed

set XACT_ABORT ON

begin tran

...........
insert......
select.....
update.....
...........

commit

If you run it and one or more statements in the transaction will failed - it does ROLLBACK automatically all that was in the transaction. Without it - actions that they can succeed -  succeed and the others will failed.
The default value of XACT_ABORT is OFF, that why we have to activate it.