According to Wikipedia:
Various studies confirm that a large majority of human species (over 90%) is heterosexual or “straight”. Straight people feel innately attracted to members of the opposite gender for matters of romance, marriage and sex. On the other end of the spectrum are homosexual or “gay” people, who feel attracted to members of their own gender. Female homosexuals are also called lesbians. The term bisexual is used for people who fall somewhere in the middle of this continuum. And a number of people also consider themselves asexual, i.e. they have no romantic or sexual inclinations for anybody.
A helpful way to understand sexual orientation is the example of handedness. Although most people are indeed right-handed, there is also a smaller group of people who are naturally left-handed or are ambidextrous. (Ambidextrous people have an inclination for both hands or to a different hand in different situations.) Different human cultures and educational systems deal with left-handed people in different ways, ranging from harsh punishment to total acceptance. (Incidentally, lefties may have an advantage with Arabic, which is written from light to left!)
It is also important to distinguish between homosexual personal identity (and the culture associated with it) on one side and homosexual acts on the other.