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).
Before the reclamation of 'cissexual' in the English language, most labels for sexual orientation relied on a binary understanding of gender, which created problems for those who are nonbinary or questioning. The reclamation of the term "cissexual" largely erased this problem, as it is able to describe whether or not one's sexuality falls within the assigned-at-birth gender binary.
Most common labels
Heterosexual / Straight
Attracted to those of the opposite gender/sex.
Homosexual / Gay (non-woman) / Lesbian (non-men)
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.
A person who is exclusively attracted to trans/genderqueer people