Cryotherapy (Liquid Nitrogen)
In this technique, liquid nitrogen (which has a boiling point of only minus 200°C)  is applied to the skin with the purpose of destroying surface-based growths.  It is therefore suitable for the treatment of a wide variety of benign skin lesions, including pre-malignant actinic keratoses, also known as solar keratoses. 
In most cases, it is not suitable for the treatment of genuine skin cancers though on occasion it may be used via an adaptation of the usual technique, for superficial Basal Cell Carcinomas (BCC), and superficial Squamous Cell Carcinomas (SCC).

It is absolutely contraindicated in the treatment of most subtypes of BCC, invasive SCC and Melanoma.

Unfortunately, because of the availability and ease of use, it is frequently used inappropriately.  Be certain your doctor has the correct diagnosis before consenting to liquid nitrogen therapy.  If used on inappropriate skin cancers it is not only unlikely to be ineffective, but may lead to a significant delay in the correct treatment being commenced.