STD is an abbreviation for sexually transmitted diseases (STD).
Sometimes you can also find other terms used in place of STD, such as sexually transmitted infections (STI) and venereal diseases (VD).
All three terms reffers to illnesses that have a high probability of transmission by means of sexual behavior. Importantly, sexual behaviours includes apart from "traditionall" vaginal intercourse also oral and anal sex. However, some of STDs can also be transmitted through childbirth or breastfeeding or even via the use of "shared" drug needles. More and more often term "sexually transmitted diseases" as well as term "veneral diseases" is replaced by term "sexually transmitted infections", mostly because of its broader range of minning. To give you an example a person who caught an infection might not have disease (or its symptoms) but he or she may potentially infect others.