Citric acid is a tricarboxylic acid with the formula C6H8O7.H2O occurring in plants, especially citrus fruits (in lemons up to 8%) and in animals. The commercial product melts at 153oC. Its main uses are based on its being a tricarboxylic acid.
Lactic acid C3H6O3, 2-hydroxypropanoic, CH3CHOHCOOH, is a hygroscopic alpha-hydroxy acid is the primary acid component in sour milk. Since it contains both hydroxyl and carboxyl functional groups it may undergo self-esterification and polymerization. It occurs in three optically isomeric liquid forms: L form, in blood and muscle tissue as a product of glucose and glycogen metabolism; D form, obtained by fermentation of glucose; and DL form, a racemic mixture present in foods prepared by bacterial fermentation, and also made synthetically.
For more information consult any organic chemistry book.