Questions about fsync

In the context of PostgreSQL 7.4, running over Linux Suse 9.0, I have some
doubts that wanted to evacuate:

In which processes impacts the property fsync?

Which can be the problem to use fsync in false?

When use fsync = false, the effective recording of the data in disk is done
for criteria of the operating system, ¿ Is exact?

Considering fsync = false, checkpoint_timeout=300.
It given a crash of the operating system, after normally db shutdown.  At db
startup, there was corruption of data and a pg_resetlog
 was done.

It is possible to lose data changed previous to last checkpoint ?

Thanks,
Fabio