It is basically an interference pattern. Wikipedia has a decent explanation of it (with formulas fir extra fun). Typically, you can reduce/eliminate it by slight changes to your position from the subject but sometimes it is pretty persistent.
I think there are plugins to reduce it but as you would imagine, they can be pretty heavy handed and you lose alot of detail. Best to avoid it.
Vidalgo,
Honestly, I don't really see it in the rock shot. Or at least it isn't something that couldn't be confused/passed off for natural variance in the rock texture and color. But, I imagine it can happen with fine details of all sorts, it is just alot easier with parallel lines which interfere easier.
The skin example is not as common in my experience but makes sense. If you look at your skin (particularly guy's skin and particularly on the hands) you have pretty visible lines. Definitely a bigger problem in fashion work where you are always dealing with fabric. It can totally destroy a good bird shot too.