A condom is a sheath of very thin latex rubber, which must be rolled onto the erect penis.  A condom prevents pregnancy by blocking the passage of semen and is effective only when used from "start to finish"  during every act of intercourse.

Contact between the penis and vagina must be avoided before the condom is put on, which should be done well before intercourse. After ejaculation the penis (together with the condom) must be withdrawn before the man loses his erection, otherwise the condom may slip off into the vagina.

Condoms have one particular advantage over all other contraceptive methods.  When correctly used, they provide almost 100% protection against sexually transmitted diseases.   They form a barrier to the bacteria and viruses which causes such diseases as gonorrhoea, syphilis, chlamydia and AIDS. AIDS is by far the most serious of these, although not the most widespread. It is a fatal disease and infected people can transmit the AIDS virus without being ill or without even knowing that they are infected.

Condoms come in many varieties of shape, size and colour, with or without lubricant coating and are on sale in most stores.

For more information contact your local health department or visit these websites.
Planned Parenthood
MayoClinic