Actually, it's not only about ACL it's a lot more than that!! See, ACL is just a base feature that a router/switch/FW uses. If you want a general thought about FWs, then it simply, filter, route, and/or translate traffic! Beside other things they can do such as VPN terminations or simple IDS devices. Start by readying about general Security concepts before you go to the PIX firewall so you can fully understand its potential design, management or deployments.