A software bug is an error, flaw, mistake, failure, or fault in a computer program.
Nickm of the Tor project had this to say on the source of bugs in December 2008 after being thanked for fixing a bug regarding the literal parse of an address in dns_resolve_impl:
"I am always glad when people are polite enough to act as though bugfixes come from programmers, and bugs come from nowhere in particular. Thanks for your kindness. ;)"
gdb is essential in order to hunt down and exterminate computer bugs.