Endogenous ketones are produced by the liver through the breakdown of fatty acids. These ketones are produced and used as a fuel source when carbohydrate consumption is limited. The process of producing ketones is called ketogenesis and it puts the body in a metabolic state of ketosis.
Exogenous ketones are produced outside of the body and consumed. Exogenous ketones are comprised of BHB. R-BHB (also called D-BHB) has been shown to be more ketogenic than the S/L enantiomer. 
Consumption of exogenous ketones can increase blood ketone levels and enhanced the benefits of ketone bodies. 
Exogenous ketones are metabolized similar to endogenous ketones but do not impair endogenous production unless consumed in large quantities.