The term homophobia was originally used to describe "a fear of men, or aversion toward the male sex." But in the late 1960s George Weinberg, a therapist, began using the term to describe a fear and hatred of people who were gay, or homosexual.
The term phobia can be a bit confusing as it sounds like some sort of hard-wired fear response that people are just born with, even though that's not the case. But it makes sense that there is so much homophobia given that most mainstream societies teach directly or indirectly that to be gay (or lesbian) is wrong.
Today the term homophobia is used in many ways to describe not only a fear or hatred of people who are gay, but also the belief that people who are gay, lesbian, and bisexual are inferior to heterosexual people; that there is something wrong and bad about being gay, lesbian, or bisexual. Homophobia isn't just explicit acts of violence or hatred it is also everyday negative attitudes and feelings about people based on their sexual orientation.
Homophobia is directly connected to heterosexism and heterocentrism, which are pervasive belief systems that say that the "natural" or "normal" way to be is heterosexual, and that anything other than heterosexual orientation or identity is evil, immoral, sick, and wrong.
Read more about homophobia: