How a Tattoo is done
While the tattooing process is age-old, as technology has advanced, sophisticated machinery has replaced the crude single needle with ink used to tattoo. Basically, skin has two layers, the epidermis which changes and renews with time, and the dermis which never changes. In the normal body, the epidermis protects the dermis from injury, scarring, and staining. The body can then get tanned, coated, painted, etc, without permanently changing the dermis. A change in the dermis, otherwise known as a permanent scar, is usually to be avoided. Tattooing bypasses the protection provided by the epidermis, and allows a permanent design to be embedded in a layer that cannot be removed by the normal healing process of the body.
During the actual tattooing process, the ink is injected in little pin-points basically with one ink dot per stick. In the distant past, sailors would take weeks to complete a design (since they were at sea for months at a time, that was not an issue). Now, with the aid of electricity and modern technology, the tattoo needles are sophisticated, and are able to stick the dermis hundreds of times a second, and needle bundles can do larger areas at one time. Despite the change in the speed of the process (elaborate designs can be done in a fraction of the time it took years ago) it is still just a stick and a tiny drop of ink.
There is a limit on how much injury the skin can take in one session. This is why elaborate designs take multiple visits to complete. The skin has to heal. As it heals some of the ink is taken away by your bodies immune system and the tattoo fads and blurs into what it eventually will become. The trauma to the skin takes months to fully heal as specialized cells carry away the excess ink. The multiple skin sticks need to heal as well and so there will be swelling and tenderness for days to come.