Sexual identity relates to one's sexual and romantic relationships with others. Here are some popular terms that help describe (not define) sexuality. Labels used well have no boundaries between them, they are just intersections of people with similar experiences.
NOTE: I see labels as a rallying point to find people like yourself, not as a box to limit you. Positive uses for labels include exploring ideas and themes around your identity, knowing what words to google and finding people to talk to.
Heterosexuality is romantic attraction, sexual attraction or sexual behavior between persons of the opposite sex or gender.
Bi-Alliance uses the term "bisexual" to refer to someone who experiences attraction (one or more of the following kinds: sexual, romantic, emotional, or affection) to more than one gender.
Homosexuality describes romantic or sexual feelings or behaviour between members of the same sex or gender.
Pansexuality describes sexual or romantic attraction towards people regardless of their sex or gender identity. Pansexuals find gender and sex irrelevant in determining whether they are attracted to others.
There is debate regarding whether bisexuality excludes complex or non-binary genders or whether the word is implicitly inclusive. The word "pansexual" is explicitly inclusive, which some people prefer.
Asexuality is a lack of or low sexual attraction. Generally it does not imply a lack of romantic feelings, although the term is an umbrella term that can cover a range of asexual sub-identities.
Demisexuality is a sexual orientation in which someone feels sexual attraction only to people with whom they have an emotional bond. Most demisexuals feel sexual attraction rarely compared to the general population, and some have little to no interest in sexual activity.