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).
In English, most labels for sexual orientation rely on a binary understanding of gender, which can create problems for those who are nonbinary or questioning.
Most common labels
Heterosexual / Straight
Attracted to those of the opposite gender/sex
Homosexual / Gay (male) / Lesbian (female)
Attracted to those of the same gender/sex
Bisexual
Attracted to 2 or more genders
Polysexual
Romantic/sexual attraction to some, but not all, genders.
Pansexual
Not limited in sexual choice with regard to biological sex, gender, or gender identity.
Asexual
A person who feels little to no sexual attraction to anyone.
Aromantic
A person who is aromantic does not experience romantic attraction to anyone regardless of gender. This person does not have to identify as asexual & they still may experience sensual & aesthetic attraction.