Sexual Orientation refers to what gender(s) or sex(es) a person can become attracted to. The term is usually used to encompass multiple forms of attraction but in some cases refers purely to sexual attraction wherein a person's orientation is split into two parts: romantic orientation and sexual orientation. This is especially useful for people who experience a difference in these two aspects of attraction (something common with asexuals in particular).
Most common labels
Heterosexual / Straight
Attracted to those of the opposite gender/sex.
Homosexual / Gay (male) / Lesbian (female)
Attraction to those of the same gender/sex.
Attraction to 2 or more genders.
Romantic/sexual attraction to some, but not all, genders.
Attraction to all genders, however, gender is a factor in attraction.
Not limited in sexual choice with regard to biological sex, gender, or gender identity.
A person who feels little to no sexual attraction to anyone.