What are chains?

Chains are blocks of rules that can act upon a packet
Rules can inspect, alter and make decisions about a packet
Rules are processed sequentially until the packet encounters a rule that ACCEPTS or DROPS it
Packets may be acted upon by several rules
A rule may tell a packet to jump to another chain
The chain's POLICY is applied if no rule ACCEPTS or DROPS the packet

Chains are blocks of rules that can act upon a packet