A hair iron or hair tong is a tool use to change the structure of the hair with the help of heat. There are two general kinds: Straightening irons, used to straighten the hair, and curling irons, used to make the hair curly. Straightening irons may also be called flat irons, but not to be confused with clothes flat irons. There are also irons, called styling irons, that can be used to both straighten and curl the hair. One tool means that you can straighten one portion of the strand and curl the other.