Henna tattoos are an ancient custom in India: girls and women are body painted before ceremonies, like marriage. They painted complicated patterns on their hands and feet to symbolize their fertility.
Henna (or 'Mehndi', the Indian name) is made from the henna plant (Lawsonia Inermis). The leaves of the plant are dried and ground. The powder gets mixed with water and you get a sort of green-brownish mud. If you put that mud on your skin, let it harden, and then peel it off, you will notice it has left an orange color.
You can get your henna at an eastern shop or a 'souk' in many different colors. There's orange, mahogany, brown and black. The orange one is the traditional henna. The black henna is synthetic. It contains PDD (P-phenylenediamene) and can cause allergic reactions. Even if it doesn't cause irritations, it's still unhealthy. Moreover, in the past, black henna at your wedding was considered a disgrace.