In the process of scarification, scars are formed by cutting or branding the skin. There are many reasons why people may turn to scarification. Aesthetically, scarifications are usually more visible on darker skinned people than tattoos. Also, unlike tattoos, scarifications are a product of one's own body.
There are also religious and social reasons for scarification. According to some tribal belief in Africa, producing scars on newborn children helps preventing vision related illness. There may also be religious expressions used in the scarification process.
Scarification is not a precise art; there are many variables, such as skin type, depth of the cut, and how the wound is treated while healing, that make the outcome somewhat unpredictable.
The body creates the scar, not the artist; it is important to keep in mind that a method that works well on one person may not work so well on another. Also, the scars tend to spread a bit as they heal, so scarifications are usually relatively simple designs -- small details can easily get swallowed up in the healing process.