The acronym STD stands for sexually transmitted disease. STI stands for sexually transmitted infection, which is basically just another term for STD. An STD is a disease that is transmitted from one person to another through sexual activity or, more specifically, contact between sexual fluids like semen and mucous membranes associated with sex (genitals, mouth, etc.). But some STD's can also be transmitted through non-sexual means like blood transfusions or sharing hypodermic needles.
Most STD's are bacterial or viral in nature. Some of the more common bacterial diseases include chlamydia, gonorrhea, and syphilis. Viral infections include herpes, hepatitis, and HIV (AIDS). The biological outcomes of these diseases are obviously all bad, but some are worse than others. Chlamydia mostly leads to inflammation in the pelvis and genitals and affects urination and menstrual cycles. It can lead to pregnancy issues with women, including infertility, but it can be cleared with antibiotics. Gonorrhea shows many of the same symptoms (mostly related to inflammation and burning in the pelvis and genitals, and can also be effectively treated. Syphilis presents its symptoms in four different stages that increase in severity and range from skin ulcerations and rashes to CNS (central nervous system) and heart issues. Syphilis can also be treated with antibiotics, but it is much easier to treat in the early stages than in the late stages.
The viral infections are generally more dangerous in that there is no true cure for them. The best hope is to control the symptoms through medication. Herpes produces cold sores on the mouth and genitals and also fever and flu-like symptoms. Once a person builds up antibodies to the virus, future outbreaks can be reduced and controlled more effectively with medication. Hepatitis includes an entire family of viruses. Hepatitis B can be transmitted sexually or through the bloodstream. It affects the liver and can cause inflammation or even cirrhosis. These is no cure but there is a vaccine to prevent contracting it in the first place. HIV is the worst of the STD's. There is no cure and it will eventually lead to contracting AIDS, a life-threatening disease that weakens the immune system to the point of death. There are several drug combination regimens (often called "cocktails") available on the market to help lessen the symptoms and prolong an infected person's life, but there is no cure and the virus is ultimately fatal.