Atypical moles and dysplastic nevi are acquired melanocytic
lesions of the skin whose clinical and histologic definitions are
controversial and still evolving. Numerous definitions and criteria
have been proposed, including the use of the term "atypical moles" for clinically
abnormal nevi and "dysplastic nevi" for histologically abnormal nevi.
Unfortunately, when clinically abnormal nevi are evaluated
histologically, some studies have shown a lack of concordance, with
some clinically abnormal nevi having no dysplastic features and some
normal-appearing nevi having some dysplastic features.
For a good discussion see: ATYPICAL MOLE in eMedicine.com.
The term "Ugly Duckling" was proposed in 1998 by two French dermatologists to help observers discern those nevi which are worrisome. The UGLY DUCKLING (UD) is the one that is not like his brothers.
Nevi in patients generally resemble each other. UD nevi stand out and can be identified by dermatologists, indeed by most observers. These lesions require attention and possibly biopsy.
A recent article in the Archives of Dermatology, "The "ugly duckling" sign: agreement between observer" discussed the utility of Dermoscopy in helping to diagnose worrisome moles. It can be found here in pdf: Download dysplastic_nevus.pdf
Another important article on Atypical Nevi appeared in the European Journal of Dermatology in 2006:Download burgdorf_pdf.pdf
It is a bit on the long side, but worth reading for anyone who wants to learn about this topic.