Henna is a plant used in Morocco and many other countries/cultures to dye the skin in intricate patterns, sometimes called temporary tattoos. In Morocco, it is quite common to see henna on women's hands and feet for weddings, special occasions, or even just for a treat, while men use it on their hands and arms only for extremely special occasions. About 1 in 20 women that I see is wearing henna, while I have never seen a man with it. Of course, if the man's arm is dyed, it would be hidden, since Moroccan clothing nearly always has long sleeves.
Henna has something of a religious importance in Islam (although I've been told that real tattoos are forbidden) and there is a whole ceremony that goes along with its usage. Women gather together and sing or chant while the henna is being applied, and eat special foods that go with the henna spiritually. After the first woman is hennaed, the other women at the ceremony each apply a bit of henna as well.