A condom is a barrier device most commonly used during sexual intercourse to reduce the probability of pregnancy and spreading sexually transmitted diseases (STDs—such as gonorrhea, syphilis, and HIV). It is put on a man's erect penis and physically blocks ejaculated semen from entering the body of a sexual partner. Because condoms are waterproof, elastic, and durable, they are also used in a variety of secondary applications. These include collection of semen for use in infertility treatment as well as non-sexual uses such as creating waterproof microphones and protecting rifle barrels from clogging.
You can read more about the history and how to use it here.
Lets talk about some other uses of condom besides wearing on your little boner. The first one was my favourite, it's so cute!