Necrobiosis Lipoidica is a degenerative disorder of collagen, which manifests itself with plaques that are typically erythematous to yellowish in the center, fading to erythematous to violaceous at the periphery. The usual age on onset is in the thirties, it usually (but not always) accompanies glucose metabolism dysfunction, and it is more commonly than not on women. The most common laboratory abnormality is an abnormal HgbA1C, but often they also exhibit an elevated fasting blood glucose.
The lesions can ulcerate as a result of trauma, and can be precipitated by trauma. The clues to diagnosis are location (pretibial), color (that characteristic yellowish color in the center of the lesion gives it away) and the pertinent negatives: it is not ulcerated, it is not focal like an infection and it is not elevated, as in pretibial myxedema. It is not annular, like GA, either. And, unlike Erythema Nodosum, it is multicolored and shiny in the center, as opposed to EN which is more focal, usually uniformly red and, if anything, slightly raised. This is a bit atrophic.
As mentioned above, it is more common in diabetics, and used to be known as Necrobiosis Lipoidica Diabeticorum, but now, since it is not just associated with diabetes, it is simply known as Necrobiosis Lipoidica.