Reg - Change notification

Hi,

Let me excuse if this is not the correct list to throw this question as
follows.

When any data in the table (actually in the row) is changed, the relational
data integrity (defined through the parent key - foreign key)
constraint/relationship will take care of updating the other tables. Thus
maintaining the consistency across the whole database.

My questions are:
1. How it is notified to the outside world that some data has been changed
in the table? What is the type of mechanism of this reporting in the
postgreSQL database??

2. What is the granularity of this change notification. Wether it will tell
the exact attribute that has changed or it will give the name of the table
and the index of the row in which the change has occured.

This two questions are especially important in the situation where there is
a database client application working with the postgreSQL database server,
and the requirement is that the data image at database server side and the
client side should remain consistent at any point of time.

thanks in advance.
Regards,
Anil.