HALT does 2 things.
(1) it halts what ever script was running, returning control to the user, so if you ran alias bob and that had run alias fred which was running alias bill, and bill did a halt, you get control back
(2) if its in a remote event or an alaias called by a remote event, it does (1), if the remote event had a ^ prefix on it then the defualt action of the remote event wont happen.
HALTDEF does 1 thing.
It does option (2) above without option (1)
PS : the other reply was correct you should uses /HELP /HALT to look up help on this, although help on this subject isnt extensive., so i replied (even tho i didnt say much more)