Plan-B is not an abortion and does not prevent STD transmission. It is emergency contraception, using readily available birth control pills/oral contraceptives. It need to be prescribed by a MD, PA, or NP. The sooner it is taken after contraceptive failure/unprotected intercourse, the better likely it will work. It is basically a mega-dose of OCs. There can be serious side effects, and is not for everyone (as no medicine is). Be prepared. Be safe.
For more information, here is a good website which includes examples of what types OCs can be used, side efffects and other advice.
Pregnancy can be prevented after intercourse by taking Emergency Contraceptive pills (EC). It works by giving the body a short, high, burst of synthetic hormones. This disrupts hormone patterns needed for pregnancy. EC disturbs the ovaries and the development of the uterine lining, making pregnancy less likely. EC is used within 120 hours (5 days) after intercourse. It is most effective within the first 24 hours. Emergency Contraception reduces the risk of pregnancy by 75%. EC does not protect against reproductive tract infections, including HIV/AIDS.
http://www.fwhc.org/birth-control/ecinfo.htm