What is a Smurf attack, and how does it exemplify a DDoS attack?

A Smurf attack is a type of DDoS attack that relies on ICMP (ping) packets. The attacker sends ICMP echo requests to a broadcast address, spoofing the source IP address to be that of the victim. All hosts on the network then respond to the spoofed address, flooding the victim with ICMP echo replies. It's like shouting a question to a large group of people, but telling them all to shout the answer directly to one specific person.