One of the most common sexually transmitted viruses, HPV can infect the genital area or other mucous membranes (such as the mouth). This highly treatable virus can also be present, but asymptomatic. Symptoms of the condition include genital warts, which are cauliflower-esque lesions. These appear either on the surface of the genitalia, or on the vaginal wall, or in the cervical tract itself.
Epidemiologists have classified these lesions in two major categories: high-grade and low-grade. The distinction relates to the statistical likelihood that the lesions will develop into cervical cancer. Certain types of HPV lesions, if left untreated, often will become cancerous. This is why it is so important for women to get regular Pap smears to detect any cytological abnormalities in the cervical tissue.
Most of this information comes from talking with my colleagues at the McGill Department of Epidemiology. They're only my colleagues because I fetch them scientific articles and coffee. Feel free (if you're a specialist) to yell at me for being inaccurate. :)