For ASCII-art fans, the chains are arranged like so: (Note: this is a very
different arrangement from the 2.0 and 2.2 kernels!)

                          _____
Incoming                 /     \         Outgoing
       -->[Routing ]--->|FORWARD|------->
          [Decision]     \_____/        ^
               |                        |
               v                       ____
              ___                     /    \
             /   \                  |OUTPUT|
            |INPUT|                  \____/
             \___/                      ^
               |                        |
                ----> Local Process ----

The three circles represent the three chains mentioned above. When a packet
reaches a circle in the diagram, that chain is examined to decide the fate of
the packet. If the chain says to DROP the packet, it is killed there, but if the
chain says to ACCEPT the packet, it continues traversing the diagram.