Loggin a user who changed something in cetain table

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