The units of measurement mg and ml stand for milligrams and milliliters, respectively. Milligrams measure an object's mass, while milliliters measure the volume of liquids. The prefix "milli" means a fraction of a thousand, which means milligrams (1/1000 gram) and milliliters (1/1000 liter) measure very small quantities.
These units are part of the metric system of measurement which originated in France at the end of the 18th century. Milligrams are mostly used in medicine to measure the dosage intake of tablets and pills, whereas milliliters are used in cooking to measure a liquid ingredient's volume, and in selling small- to medium-sized commercial beverages.