The fork bomb is a form of denial of service attack against a computer system that uses the fork function. It relies on the assumption that the number of programs and processes which may be simultaneously executed on a computer has a limit.
A fork bomb works by creating a large number of processes very quickly in order to saturate the available space in the list of processes kept by the computer's operating system. If the process table becomes saturated, no new programs may be started until another terminates. Even if that happens, it is not likely that a useful program may be started since the instances of the bomb program are each waiting to take that slot themselves.
Not only do fork bombs use space in the process table, they each use processor time and memory. As a result of this, the system and existing programs running on it become much more difficult (slow), or even impossible, to use.
Fork bombs can be considered as a special type of wabbit (a program that self-replicates without using hosts or network functionality).