ACLs are what tells the router if data is allowed to pass through the router or not. For example if you only want people to connect to your web server on port 80 then you would deny all other ports and allow port 80.
QoS tells the router which packets are more important than others. For example you would want to have a high QoS for Voice Over IP and a very low QoS for streaming video.
Packet Filtering is when the router looks at the actual TCP/IP packets looking for invalid data such as a DoS (Denial of Service) attack and will simply drop the offending packets instead of forwarding them on to the end machine.
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