Basically, FORWARDed packets pass through the firewall box without a local destination, ie they don't start or stop there. However, packets that go along the OUTPUT chain start on the firewall box and packets that go along the INPUT chain finish on the firewall box.
As an example, if you provide internet access to your users without a proxy server, the packets use the FORWARD chain to go through your box to the remote web server and back again. When they use your proxy server, the packets arrive at the firewall/proxy and are processed on the INPUT chain and given to your proxy server. It then sends them to the remote web server from the firewall's OUTPUT chain.
For a better explanation, there's a useful document at http://iptables-tutorial.frozentux.n...ERSINGOFTABLES
that covers the path that packets take through the various chains under different scenarios.
For PREROUTING, have a look at http://iptables-tutorial.frozentux.n...REROUTINGCHAIN
Hope that helps...