This is the famous Mani mantra of lovingkindness, also somewhat of a protection, pronounced Om Ma Ni Pehme Hum.
The Tibetan letters here (often mistaken for Sanskrit) have been highlighted with the vowel signs in red, there is also an addition of a 'heading' character the double swirls at the start of the mantra.
Design by Tibetan calligraphy artist Tashi Mannox: www.inkessential.com
Very nice calligraphy!
True, this is not Sanskrit, but the reason people confuse is because they do have things in common, the horizontal line, and one can see similarities in many characters like the na and ma.