Doubled die is a (relatively common) mint error that results in a double image which is most likely to be on the writing or date of the coin. Unlike regular mint errors such as being off center or missing certian items, the doubled die is more common and usually specific to certain years. Though no not as valuable as most mint errors, doubled die coins are sometimes double or even triple their regular value. These error are not specific to one type of coin but, they are more common and popular in pennies.
Doubled Die Obverse, Reverse
A Doubled die obverse is on the front side of the coin as shown above. A Doubled die reverse is on the back of the coin as shown below. (Image needed)
As you can see in comparison with the obverse the reverse doubled die is much less noticeable.