What are homosexual persons? What phenomena leads the formation of a gay man (a person whose sexually attraction is towards the person of same gender)?

A homosexual person is the one who is sexually attracted towards the individuals of same sex. The most common term for a homosexual male is gay, while for females it is lesbian. 

Sexual orientation is not governed by specific factors. It is a result of a complex interactions between the genetic, physiological and psycological factors. So, it is a purely natural phenomenon and homosexual orientation is as normal as bisexuality or heterosexuality.