Hair cutting or hair trimming is intended to create or maintain a specific shape and form. There are ways to trim one's own hair but usually another person is enlisted to perform the process, as it is difficult to maintain symmetry while cutting hair at the back of one's head.
Cutting hair is often done with hair clipper, scissors, and razors. Combs and hair grips are often employed to isolate a section of hair which is then trimmed.
Read more, here.