Sexual abstinence (often referred to as just "abstinence") is the practice of not engaging in sexual intercourse. The most common reasons for doing this are contraception (i.e. preventing procreation), and preventing the transmission of sexually-transmitted diseases.
Abstinence is often referred to as the only contraceptive that is 100% effective and 100% safe; this is arguably false. When used as a contraceptive tool, sexual abstinence is actually planned abstinence, as its success depends not just on abstinence itself but also on adherence to one's plan to use it.
Some groups suggest that sexual abstinence should be taught and promoted as the best (or only) method of contraception; such groups are generally described as pro-abstinence, and either favor abstinence-based education or are against sex education altogether.