One of the class wrote to me after our Tattoo session yesterday to ask if I thought Henna designs might be considered as tattoos. I replied that in some ways they were, but in our email exchange, it became clear that the overriding characteristic of tattoos is in their Intentionality; that is, they are meant to be permanent, even if, through considerable laser treatment, they can be erased, creating a palimpsest of the body. As far as a tattoo's materiality is concerned, it is the embedding of ink under the skin that provides that permanence, and this is quite different, of course, from body-painting. It is this breaking of the skin and embedding of substance that aligns tattoos with piercings. Ultimately, of course, tattoos and any other form of 'permanent' body text are evanescent, transient, ephemeral.
From the functional perspective, tattoos are both private and public in nature. Some are literally so, because you cannot see them, unless the person chooses to show you. Some are intellectually private, in that their meaning is not obvious. This might be because of the language in which the tattoo is written (as in the textual examples here, http://www.englisc-gateway.com/bbs/topic/25675-anglo-saxon-early-english-tattoo-designs/), or because the immediate reference is obscure. The ways in which viewers or readers of the body react to the tattoo is another aspect of functionality: how do tattoos function in the world, and is the tattoee's intention sometimes to shock? It is this former aspect of tattooing--the counter-hegemonic, the rebellious--that might parallel the more public act of writing graffiti.