In this video on Dave's Garage, he discusses the "fork bomb," a DoS (denial-of-service) attack where a process replicates itself until it depletes all system resources, slowing or crashing the system. The bomb is nothing more than an 11-character program that, when entered into a Bash prompt, will replicate itself until the computer crashes.
Dave demonstrates the carnage a fork bomb can do and then shows you how you can protect your machine(s) from such a malicious attack.