Tattooing is a process in which a needle impregnated with ink is repeatedly used to pierce the dermis, leaving the ink embedded in the skin to create a design. Modern tattoos are done with "guns" that are essentially motors attached to a series of needles. The artist presses a foot-pedal and activates the machine. Traditionally, a tattoo would have been applied with a needle made of bone, metal, or other sharp natural material. The needle would have been affixed to a short wooden handle which would have been rested on a cushion so it hovered a few centimeters above the skin. Another stick would have been used to tap the back of the needle and drive it into the skin and push the ink into the skin. The word "tattoo" is a loanword from the Polynesian "tatau", for the sound the traditional tattoo instruments make.
Once a Tattoo is applied to the dermis it can only be removed via laser surgery, though many can be covered with other tattoos and worked into other designs by a skilled artist. The most popular tattoos are of religious imagery and memorial tattoos.
Ye shall not make any cuttings in your flesh for the dead, nor print any marks upon you: I am the LORD. Leviticus 19:28 (KJV)
However this quote forbids the specific practice of cutting one's skin and rubbing charcoal or other black material into the wound for the purpose of mourning the dead; it was a religious act that was found amongst many tribal societies at the time. The modern idea of tattooing for decoration and self-expression did not exist at the time.
Getting a tattoo is not a threat to human health provided the artist follows safety guidelines that are often rigidly enforced by the state they practice in. Back-yard tattooing is far less safe because of the risk of contamination of the instruments.
A properly outfitted tattoo shop should have biohazard containers for objects that have come into contact with blood or bodily fluids, sharps containers for old needles, and an autoclave for sterilizing tools. Needles should come in sealed packages and should be opened only when they are needed by the artist. Tattoo machines themselves are often covered with plastic to protect the machine and the patron. The artist should wear gloves at all times and should wash his hands before and after working on a client.
For the safety and comfort of the client, a good tattoo artist will generally refuse to tattoo people while they under the influence of drugs or alcohol.
The instructions given by the tattoo artist after receiving a tattoo are vital because if a tattoo heals improperly, it will distort and possibly discolor. There is no standard for the best aftercare methods, but it is generally agreed that a person should avoid excessive contact with water (soaking) a new tattoo while it is still scabbing and picking or otherwise trying to remove scabs should be avoided.
There are several products on the market designed for aftercare of a new tattoo, but there is no indication these work better than soap and warm water for keeping the tattoo site clean.
In recent years, tattoos done with special inks that fluoress under UV lighting. These tattoos are difficult (but not impossible) to see under normal lighting conditions. UV ink is a popular ink to highlight already healed tattoos however the ink is difficult to work with because the color is not as bright and it doesn't mix well with other tattoo inks.
Some adverse effects, including skin irritation and redness, have been reported but by and large there is no indication that UV inks pose any threat to health.