HPV is short for the "human papillomavirus", which is a family of viruses that infect epithelial tissues such as the skin and mucosa. There are over 200 known strains of the virus, roughly 40 of which are known to be sexually transmitted. The sexually transmitted strains of the virus fall into two broad categories, "high risk" strains and "low risk" strains. Most infections, with any strain, are asymptomatic but infected persons may still be contagious until the infection clears--usually around two years post-infection.
Symptomatic infection with "low risk" sexually transmitted strains usually manifests as condylomata acuminata (genital warts), most commonly found on the vulva or the anus. Warts can also occur inside the vagina or (very rarely) inside the uterus in women. Men commonly develop warts within the urethra, however such cases are unlikely to be discovered unless the warts lead to sexual dysfunction or painful urination.
Chronic infection with "high risk" strains may lead to the development of cervical, anal, or penile cancer. Because these cancers take many years to develop (and can have many other causes), regular screening for cervical cancer is advisable even for women who consider themselves to be within a low risk population.
HPV is the most prevalent sexually transmitted disease. Population studies testing for the presence of antibodies against viral proteins suggest that around 75% of American women will be infected with at least one sexually transmitted strain of the virus at some point in their lives. These numbers are surprisingly consistent across all demographics examined.
Merck has developed a vaccine against the four most prevalent sexually transmitted strains of HPV; see Gardasil. GlaxoSmithKline later released its own vaccine, Cervarix, which protects against the two most prevalent high risk strains. Both vaccines are manufactured using recombinant viral capsid proteins.