Hair and Nail Formula is a formulation designed to provide the nutrients healthy hair and nails require. In addition to a high dose of Biotin, it also contains N-acetyl cysteine, PABA, inositol, and Silica.
Biotin is a B vitamin and renews the follicles and 
roots of hair, resulting in thicker, fuller, and healthier looking hair.
N-acetyl cysteine provides antioxidant protection of the hair follicles.
PABA is a naturally-occurring compound found in many foods as a cofactor of the vitamin B-complex. It’s known for its potential to restore natural hair color and help hair grow.
Inositol is a B vitamin and is considered one of the main nutrients to spur hair growth and prevent balding.
Silica can make your hair shiny and also help prevent thinning. It can improve circulation, which increases blood flow to the scalp to encourage growth.
Resources:
bestvitaminforhairgrowth.net/biotin-for-hair-growth/
cpmedical.net
http://www.hairloss-research.org/UpdateBestAminoAcidsforHairloss7-11.html
http://www.naturalnews.com/028378_balding_hair_growth.html
http://www.livestrong.com/article/333271-how-does-silica-help-with-thinning-hair/