Selsun Blue is available with two concentrations of selenium sulfide — 1% over the counter and 2.5% by prescription.  Selenium Sulfide is designed to prevent excessive fungus growth (Malassezia) that feeds off of the oils of the skin, thus leading to itchiness and dandruff.

Selsun Blue is designed mainly to resist seborrheic dermatitis, however selenium sulfide has also been proven to prevent the excessive production of keratinocytes, which make up most of the epidermis (the outer part of the skin), thus preventing one of the main symptoms of psoriasis.

Example of typical Selsun Blue shampoo, this particular kind contains Menthol as well to suppress itchiness (pruritus):

There are a variety of other Selsun blue variations, including types that stress natural ingredients such as Aloe Vera.