What is the difference between an ACL Router and a firewall?

Most routers allow you to set access control lists. You can say which protocols from or to which machines should be allowed. While this is an effective security technique, firewalls can do this and more.

Typically, firewalls may also provide stateful inspection (they are aware of current communications and judge new requests against those in process), dynamic packet filtering (ports are not always opened, but opened in response to requests, then closed), application layer filtering (filtering at the application vs. network layer -- i.e., based on the program, not simple packet reading, or using the application itself as a filter) and more.