A hair iron or hair tong is a tool used to change the structure of the hair using heat. There are three general kinds: curling irons, used to make the hair curly, straightening irons, commonly called straighteners or flat irons, used to straighten the hair, and crimping irons, used to create crimps of the desired size in the hair. Most models have electric heating; cordless curling irons typically use butane.
Types of hair irons 
Curling iron 
Curling irons, also known as curling tongs, create waves or curls in hair using a variety of different methods. There are many different types of modern curling irons, which can vary by diameter, material, and shape of barrel and the type of handle. The barrel's diameter can be anywhere from .5 inches (1.3 cm) to 2 inches (5.1 cm). Smaller barrels typically create spiral curls or ringlets, and larger barrels are used to give shape and volume to a hairstyle.
Curling irons are typically made of Teflon, ceramic, tourmaline, metal, or titanium, each of which has its pros and cons. The barrel's shape can either be a cylinder, cone, or reverse cone, and the iron can have brush attachments or double and triple barrels.
The curling iron can also have either a spring-loaded, Marcel, or clipless handle. Spring-loaded handles are the most popular and use a spring to work the barrel's clamp. When using a Marcel handle, named after Marcel Grateau, one applies his or her own pressure to the clamp. Clipless wands have no clamp, and the user simply wraps hair around a rod. Most clipless curling irons come with a Kevlar glove to avoid burns.
Straightening irons 
Straightening irons, straighteners, or flat irons, not to be confused with curling irons, work by breaking down the hair's hydrogen bonds found in the hair's cortex, which cause hair to bend and become curly. Once the bonds are broken, hair is prevented from holding its original, natural form, though the hydrogen bonds can re-form if exposed to moisture. They can also use infrared technology.
Crimping irons 
Crimping irons or crimpers work by crimping hair in sawtooth style. The look is similar to the crimps left after taking out small braids. Crimping irons come in different sizes with different sized ridges on the paddles. Larger ridges produce larger crimps in the hair and smaller ridges produce smaller crimps. Crimped hair was very popular in the 1980s and 1990s.