STIs
There are many STIs... here is just a selection:
(more comprehensive information can be found here: http://www.nhs.uk/conditions/sexually-transmitted-infections/Pages/Introduction.aspx)
Chlamydia
Chlamydia is one of the most common STIs, and is particularly common amongst young people. It affects both men and women, usually infecting the genitals, although it can infect the throat, eyes and rectum. Once detected it is easy to cure (4 antibiotic tablets taken at once), but because two thirds of people do not show symptoms it can easily go on to cause serious long term health problems such as infertility in women and men.
Warts/HPV
HPV (Human Papiloma Virus) is a virus that sometimes shows up as warts, which can be around your genitals or anus. There is a link between HPV in women and cervical cancer. HPV is passed on by direct contact with warts (though rarely it can be transmitted in the absence of a wart). Using a barrier method will reduce the risk, provided it covers the affected area. Vaccinations for some strains of the virus are in use but currently only some GPs will give them to sexually active people aged over 18.
Hepatitis B and C
Hep B is a highly infectious disease of the liver. Can be caught during unprotected penetrative vaginal or anal sex, through blood contact, and (though rarely) the virus is transmitted in saliva. Vaccination is recommended for sexually active gay men. Some people may carry Hep B without showing symptoms, and sometimes the virus can establish a lifelong infection. Similarly Hepatitis C is also transmitted in the blood and can also lead to a lifelong infection. Unfortunately there is not a vaccine available for Hepatitis C. (Hepatitis A, for which there is a vaccine, is transmitted through the faecal-oral route.)
Herpes
The same virus which gives people cold sores. This is passed on by direct contact with skin which has the herpes virus in it eg just before, during and after a cold sore. To prevent, use a barrier method.
HIV
HIV stands for the Human Immunodeficiency Virus. The virus attacks the body's immune system and eventually those infected with HIV can develop AIDS - Acquired Immune Deficiency Syndrome. You can't catch AIDS; only people that are HIV positive may eventually progress to AIDS. HIV is carried in semen, vaginal fluid, blood and breast milk. It can be caught by having unprotected sex, sharing needles, blood transfusions and breast feeding. There is no cure or vaccine for HIV.
Pubic Lice
Lice which live in the pubic region. These are passed on through close personal contact including sharing clothes. The only method of protection is to not get too close.
Scabies
An infection of tiny mites that are too small to see. It can easily be spread by close contact and by sharing towels.
Syphilis
This is a curable infection, if treated early enough, which you can have without showing symptoms. To reduce the risk use a condom, although this does not offer total protection as syphilis can be transmitted through skin to skin contact.