Can you please help advise about not exists and not in?

Hi expert,

I would like to know how difference between "NOT EXISTS" and "NOT IN". I have to fix some script from old DBA had been setup for monitoring db. 

===Below is machine name connect to testdb

SQL> select distinct userhost from dba_audit_trail;

USERHOST

--------------------------------------------------------------------------------

WORKGROUP\NUMNIMJUN-PC

racnode2

racnode1

===Below is list of allowed server keep in b table.

SQL> select * from b;

MACHINE

--------------------------------------------------------------------------------

WORKGROUP\NUMNIMJUN-PC

racnode2

===Assume we not allow racnode1 to connect db.

from the old script they used like this:

SQL> select distinct userhost from dba_audit_trail

  2  where not exists (select machine from b);