A Smurf Attack (named so as it fits the stereotype of Smurfs with proper visualization) is a denial-of-service (DoS) attack that involves sending ICMP echo requests (ping) traffic to the broadcast address of routers and other network devices in large computer networks with a spoofed source address (the address of the desired DoS target). Since the device receiving the original ICMP echo request broadcasts it to every other device it’s connected to, each one of these devices sends out an echo reply to the spoofed source address (the DoS target). This will generate a high rate of ICMP traffic and could cause DoS or instability for the target network.
If the original request (to a device in a large network) is broadcast to such a vast number of machines, the resulting attack can be highly effective. After 1999, however, most routers do not forward packets sent to their broadcast addresses by default, this makes the likelihood of a successful large-scale Smurf Attack fairly low.