QoS prioritizes network traffic — guaranteeing bandwidth and low latency for real-time applications (VoIP, video) over less-sensitive traffic (bulk transfers, backups).
DSCP (Differentiated Services Code Point) marks packets with priority level in IP header. EF (Expedited Forwarding) = highest priority for VoIP. AF (Assured Forwarding) = medium priority. BE (Best Effort) = default. Queue management drops lower-priority packets first during congestion. Security: mark traffic at ingress, enforce at each hop.