Tattoo is a permanent marking made by inserting ink into the layers of skin to change the pigment for decorative or other reasons (Wikipedia).
Mehndi on the other hand is an art of skin decoration using henna as the colouring agent and commonly practice by the South Asian communities during weddings, local festivals and other celebrations. The artwork is called inai by Malay speaking people in Malay archipelago and has been a long time practice by the traditional Arabs. Unlike tattoo, Mehndi is temporary and can last for 2 to 6 weeks, depending on the quality of henna and method of preparation.
I am always wondering why people go for tattoos and piercing, since I couldn't find any true beauty articulated from it. In fact it only turns them into ugly duckling, especially when tattoo is applied on their faces.
I HATE tattoo, I really do. Tattoo lovers definitely hate me for saying this. It won't change my perception no matter how good the pro-tattoo media say about tattoo arts and freedom of expression. To me, crafting something permanently on our body parts will never make us a better person, rather we will look like either criminals or morons. I'm sorry (but I accept mehndi or inai).