Locking

What is the situation with locking in pustgresql

I am aware that 6.5 has row level locking, and read-always, and that
6.4 doesnt (it just has complete table-level, if I am right).

However that is moderately beside the point.

What actually happens when I try and write to a table or row that is
locked by another instance of the backend? Does it simply wait and try
again, transparently, or do I have to handle this myself by checking on
return statuses and retrying when I get a particular status? And is this
also the case for reading a locked table in 6.4

Anyone have information on this?

Thanks