The gram is a unit of mass (acceptable for use as weight on Earth) and is a multiple of an SI base unit with the symbol g.
Although without a prefix, it actually represents 1/1000 kg. It is the first subdivision of the SI base unit; kilogram and 100 g equals 3.527396195 ounces.
It is the most common unit for measuring ingredients (except liquid) in cooking and purchasing food goods in the world today.
The majority of nutritional values and information is expressed in terms of 'per 100g'.
1 g is roughly equal to the weight of bank note or a pinch of salt.
The microgram is a unit of mass (acceptable for use as weight on Earth) and is a submultiple of an SI base unit with the symbol μg.
1 microgram is equal to 0.000001 or 1/1000000 g (one millionth of a gram).
The symbol μg is avoided in medical practises / applications because there is a chance the 'μ' could be misread as an 'm'; resulting in mg being interpreted rather than μg - which could lead to a 1000x overdose.