How to limit access only to certain records?

Hi,

is there a way to limit access for some users only to certain records?

e.g. there is a customer table and there are account-managers.
Could I limit account-manager #1 so that he only can access customers 
only acording to a flag?

Say I create a relation  cu_am ( customer_id, account_manager_id ).
Could I let the database control that account-manager #1 can only see 
customers who are assigned to him in the cu_am-relation?

For now I do this in the front-end but this is easily circumvented for 
anyone who has a clue and uses some other client like psql.


Regards
Andreas