In simple term, 'nat 0' basically tells you no address translation will be applied/applicable for the configured/stated address(es).
You can either specify the address or use access-list to define addresses to be nat exempted.
NAT 0 basically used is to allow traffic between two firewall segment without address translation, or for VPN interesting traffic (vpn via PX) where you bypass address translation to allow local internal segment to talk to other/remote segment.
NAT 0 details & example:
http://www.cisco.com/en/US/products/ps6120/products_configuration_guide_chapter09186a008052564b.html#wp1043405
http://www.cisco.com/en/US/customer/products/ps6120/products_command_reference_chapter09186a008063f0f7.html#wp1652607
HTH
AK
