Loggin a user who changed something in cetain table

[oca(at)mvc(dot)edu(dot)ph]

hi,

how can i logged users activity in the table where he is manipulating  
someting?

like if i have a table:

Table: charge_rates
       Field1, Field2, Field3, user, time

when a user login to postgre and manipulated (INSERT or UPDATE)  
something on that table, is there a way that i can record it in that  
table and specifically put the username on the record he changed?

Thank you,
oca