The Bash command :(){ :|:& };: will spawn processes to kernel death. Can you explain the syntax?

by 
:(){ :|:& };:

..defines a function named :, which spawns itself (twice, one pipes into the other), and backgrounds itself.

With line breaks:

:()
{
    :|:&
};
:

Renaming the : function to forkbomb:

forkbomb()
{
    forkbomb | forkbomb &
};
forkbomb

You can prevent such attacks by using ulimit to limit the number of processes-per-user:

$ ulimit -u 50
$ :(){ :|:& };:
-bash: fork: Resource temporarily unavailable
$

More permanently, you can use /etc/security/limits.conf (on Debian and others, at least), for example:

* hard nproc 50

Of course that means you can only run 50 processes, you may want to increase this depending on what the machine is doing!

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