µg is the correct symbol for the metric measurement microgram which is one millionth of a gram or one thousandth of a milligram.
The symbol for µ is a Greek letter (mu) which means 'small'.
Due to the symbol µ being typographically unavailable much of the time i.e. you cannot type it on a keyboard, other versions were devised; two of which are ug and mcg and are now widely used in its place.
