Nothing. Not zero (though zero may be used to represent it), but nothingness. The /dev/null device is a throw-away bit bucket used when you don't want to see the output: often a cron entry would be something like "/usr/bin/someprog > /dev/null 2>&1"
C language strings are null (zero) terminated, and functions may return NULL pointers if they were supposed to return a pointer to something real. Don't confuse this with the very common convention in the shell and elsewhere of returning 0 for success.
The fact that null is often the same as zero is a source of problems. For example, is a reference to "0" a reference to null or a legitimate attempt to access the very first byte of memory? See section 1.3 and 1.4 of http://www.lysator.liu.se/c/c-faq/c-1.html. A Google search for "NULL Pointer" will lead to all sorts of problems, confusion and errors.
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