Lactones are cyclic carboxylic esters are intramolecular esters derived from hydroxy carboxylic acids.  They can be saturated or unsaturated.  Some contain heteroatoms replacing one or more carbon atoms of the ring.
Lactones are formed by intramolecular esterification of the corresponding hydroxycarboxylic acids, which takes place spontaneously when the ring that is formed is five- or six-membered. Lactones with three- or four-membered rings (α-lactones and β-lactones) are very reactive, making their isolation difficult. Special methods are normally required for the laboratory synthesis of small-ring lactones as well as those that contain rings larger than six-membered.