Spider holes date back to World War II, the Japanese were using them to launch ambushes from. Most recently the word resurfaced in the news when the capture of Saddam Hussein became public.

Spider holes are nothing elaborate, merely just a tunnel into the ground with a vent, airhole and possibly a tube to be used as a crude urinal.

Spider holes are defined as a tight, camouflaged hiding place hence they are not intended for someone to be there for very long.

For those interested in a tour of such a hole they have become commercialized in Vietnam and there are called Cu Chi Tunnels.

update:TenMinJoe says re spider hole: Chatbox consensus is that they are called spider-holes after the trapdoor spider that leaps out of holes to ambush insects.