I learned from the Wikid answer, but I know we get it when we take a picture of a TV screen or sometimes when we scan a newspaper picture. In other words, I always recognized it, but I didn't know exactly what caused it.
Adobe says, "A wavy striped pattern in an image, resembling the pattern of watered silk. Moire can be an artifact caused by a camera’s inability to capture the detail in an image," but I'd vote for the Wiki link.
You've already seen examples, but here's something I bet you didn't think about. See http://www.flickr.com/photos/samfeinstein/1037858817/ You may notice the moire at this magnification. Click on "All sizes" and check each size. On MY monitor, I don't see it at any other magnification. You might see it on ALL sizes. The thing is, the pattern in the scan will interact with the line resolution of your monitor! I didn't realize this until today. I know that I have printed some pictures that did not show moire on the monitor and it appeared in the print. I would just take my image size up or down a bit by making a different crop until I got rid of the stripes in the print. This was baffling to me until today. Thanks for the question, because the answers taught me something. Now... Do I know how to predict or control when I will have moire problems on my output? No, but I have a better idea about controlling it.
it is a pattern that occures when the image has a repeative pattern (like a tiny checked pattern on a shirt) that exceed the resolution of the camera. You can use an Anti-alias filter to reduce this effect or eliminate it completelybut it will also reduce the sharpness of the image.
It is also a lovely satin fabric that wedding and ball gowns are made of! :-)
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