The linksys, netgear, d-link products that are called a router is not a really a router. They are mini-computers that share the internet or WAN connection, so you can hook to many computers outside your LAN. A real router does not have a firewall, but it balances the load, redirects the packets, and adds reduntancy to the network. You can categorized linksys, netgear, d-link products as smart routers. The term NAT (Network Address Translation) is the same method what a router does.
The linksys, netgear, d-link products are pretty secure if the manufacture updates the software to fix valunerablities and user checks the manufacture site to update the product. These products have their own firewall but iptables/ipfilter is much better. Though you can not have enough protection from the internet.