this is how my plugs looked when my valve cover o-rings were bad:
Bad O-rings allow oil from the valvetrain to leak into the plug bores (where the spark plug wire goes). The oil pools up in the bottom of the bores. Then when you pull the plugs out, you pull the threads and tip of the plug through the pool og oil, making it look like the plug is DRENCED in oil. The brown staining on the ceramic is from prolonged exposure to the puddle of oil in the plug bores.
The solution is pretty simple: replace the valve cover gsket and o-rings (fairly cheap)