The kernel's packet filtering system would be of little use to administrators without a userspace interface to manage it. This is the purpose of iptables. When a packet reaches your server, it will be handed off to the Netfilter subsystem for acceptance, manipulation, or rejection based on the rules supplied to it from userspace via iptables. Thus, iptables is all you need to manage your firewall if you're familiar with it, but many frontends are available to simplify the task.