This document discusses the epidemiology and classification of anal cancer. It notes that anal cancer incidence has increased in recent decades, and risk factors include HPV infection, a high number of sexual partners, HIV infection, cigarette smoking, and receptive anal intercourse. The document describes the histology and types of anal cancers, which can be squamous cell carcinomas, adenocarcinomas, or basal cell carcinomas depending on the region of the anal canal. It also covers the lymphatic drainage and risk factors in more detail.