What the wiki says:
- input – used to process packets entering the router through one of the interfaces with the destination IP address which is one of the router’s addresses. Packets passing through the router are not processed against the rules of the input chain (DST address of the router)
- forward – used to process packets passing through the router (SRC and DST is not on the router)
- output – used to process packets originated from the router and leaving it through one of the interfaces. Packets passing through the router are not processed against the rules of the output chain
