An ex-gay is someone with a homosexual orientation who typically enters a conversion program / ministry / therapy in an attempt to become heterosexual. Not all (but most) conversion programs are religiously based.

Although there is no question one's behavior and how they view themselves may change, there is a great deal of controversy if an ex-gay's core sexual orientation and attractions can ever change. Many ex-gays are little more than celibate homosexuals who continue to struggle while others are now married to members of the opposite sex and have "traditional" appearing families.

Ex-gays are disproportionally white, male and Christian. However conversion programs exist for secular and Jewish individuals as well as lesbians.

A person who goes through a Christian ministry designed to convert them from homoseuxality to heterosexuality. Most psychological and psychiatric professionals believe such ministries to be ineffective and actually dangerous to the psyches of ex-gays.

someone that used to be bi or gay

Someone who chooses to no longer have intimate interactions with those of the same gender and orientation. This person may also pursue heterosexual relationships in the process.