A hair light does exactly what it says on the tin. Use a single light without a softbox positioned well above and behind the model, pointing down to illuminate the hair. If you have more than two lights, or are using a reflector as a secondary light source, the hair light can be combined with any of the lighting styles described here.
Use hair lighting to produce a more professional and glamorous appearance for your portraits. If you have a boom arm as part of your lighting system use this to position the light directly above the model's head.