ipchains . what r they ??

From an article at http://unixgeeks.org/security/newbie/misc/ipmasq.html#toc4_5_2
(please remove any spaces)

"'ipchains' is a network packet filter as described above. It works closely with the Linux kernel to handle all network traffic arrivingand departing from a given network interface (or interfaces).

'ipchains' is initialized with just three rules or 'chains', input, output and forward. When a packet arrives at a network interface it's fate is determined by the input chain; if thepacket is accepted by the input chain, the kernel forwards the packet according to the destination address. If the destination address is another machine on the LAN, then the forward and output chains determine the fate of the packet and if the packet is acceptable to these chains, it is routed to the destination machine.

Each of these input, output and forward 'chains' are simply sets of one or more rules which can be built up - one rule at a time. Each rule examines the header informationof the packet, and if the information concerns the rule, the rule jumps to accept, reject or deny the packet. If the packet doesn't concern the rule, then the next rule in the chain is checked in the same way. Finally if no rules are relevant to thepacket, a default action would usually be set by the sysadmin to deny or reject the packet on the grounds that no rules exist to determine the packet's fate (otherwise potentially harmful packets would be allowed through).

For a masquerading firewall, it makes sense to create three sets of custom chains: one for packets arriving from the internet/external network, one for packets departing to the internet/external network and one for packets which are to be forwarded from the router to the masqueraded clients. This is the approach that we will take, using an incoming chain (inet-in), an outgoing chain (inet-out) and the built in forwarding chain set (forward)."

Joseph's answer is pretty comprehensive. I'd like to add that if you want to achieve similiar functionality in windows check out WinRoute at www.winroute.com. It's the best firewall/NAT for windows I beleive.
Though it's a commercial product.

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