what happens if something goes wrong in transaction?

["A B" <gentosaker@xxxxxxxxx>]

Hi. newbie question, but what will happen if I do

begin work;
select ...
insert ...
and so on...
commit

and somewhere a query fails. will I get an automatic rollback? If not, is there a way to get that behaviour? 
I'm using php to make all these calls and they have all to be succesfull or no one of them should be carried out.