Somehow thousands of years ago, ancient man stumbled upon the fact that breaks in the skin could be made into permanent marks by the introduction of pigment. Due to the abundance of body art on the planet, we see that trial and error led to the refinements of depth, line control and shading. Different cultures all practiced tattooing, but evolved slightly different methods for introducing pigments into the skin.

Western Tattoo
In the most common application method, a tattoo machine is used. Originally patented in England in the late 1800s, this device has changed little in the history of Western tattooing. A solid needle is moved via motor up and down in a metal tube that is dipped into a cup of ink to provide the staining material. A foot pedal controls the on and off actions. A strong hand and forearm are required to balance the top-heavy machine during long working hours. Different needle rigs can be used. Single needles or groups of needles in odd numbered rounds are used most often for outlining. Needles arranged into even numbered, flat configurations are used for shading.

Japanese Tattooing
The Japanese developed a technique for tattooing that was primarily hand based until recently. Tattoo tools were made with elaborate bamboo handles to which bunches of needles were lashes, creating a hand powered version much like a Western machine at the working end. Black ink is still ground by hand from sumi by the freshest apprentices in many Japanese tattoo shops. The tattooist stretches the skin with one hand, supporting a brush dipped in ink through the spread fingers. The active hand swipes the tip of the tattoo tool through the brush, then bracing the tool against the stretching hand, manually punctures the skin. Masters of this technique are said to have incredible depth control to their work and be able to produce unique and subtle shadings.

If you're looking for more information on how to tattoo, you might like
Tattooing A to Z: A Guide to Successful Tattooing
by Huck Spaulding
Advanced Tattoo Art (How-to Secrets from the Masters)
by Doug Mitchel