Ketones are produced in the body, by the mitochondria of your liver cells. Their purpose is to create energy. They serve the same process as glucose. If you are on a ketogenic diet, where carbohydrate intake is essentially zero your body will not have any access to glucose and will instead turn to ketones for energy. This is known as ketosis and it has many benefits.
Your body naturally produces three ketones, these are; acetone, acetoacetic acid, and beta-hydroxybutyrate acid. Because they are produced in the body, we call them endogenous ketones. Some people use ketone supplements to enter ketosis, these ketones are known as exogenous ketones because they are produced outside the body.
Taking an exogenous ketone will raise blood ketone levels and help you to enter ketosis, the supplements can also be used to maintain ketosis while on a ketogenic diet. People who train regularly while following a ketogenic diet often take exogenous ketones as they require the extra energy that glucose usually provides.
Most exogenous ketone supplements use beta-hydroxybutyrate acid (BHB) which is converted to acetoacetic acid by the body. Not all ketone supplements are actually exogenous ketones though, a popular (though completely ineffective) fat loss supplement is called Raspberry ketones, but these are not actually exogenous ketones, and do not offer the same benefits.