I was looking at the information on the front of my pack of vitamin D3 capsules and saw that they are each 10ug, i.e. 400 IU, from fish liver oils. Can't say I've noticed the 'ug' before. - Have you? - It's also known as the μg. - I knew it as the μg. - μ is a Greek letter.
So if you too are interested in the ug, aka the μg:
μg means microgram. The prefix "micro" means 1/1,000,000. Therefore, 1 microgram = 0.001 milligram = 0.000001 grams, or 1,000,000 μg = 1 g