Sexual Orientation refers to what genders you are attracted to. For example, a woman experiencing attraction primarily towards other women would be considered a lesbian.
Gender Identity refers to the personal sense of what gender you are. A cisgender person identifies as the gender they were born with, whereas a transgender person's gender identity differs with the sex they were assigned at birth.
By no means is this a complete list, but here is an overview of the LGBTQ+ community:
A lesbian refers to a homosexual woman. (A woman who is attracted to other women)
A gay man refers to a homosexual man (a man who is attracted to other men). This term can also be used as a broad term for those attracted to the same sex. (A lesbian can also be described as a gay woman, a bisexual person might refer to themselves as gay, etc.)
Bisexual describes individuals who are attracted to people of their own gender, people of the opposite gender, and non-binary individuals.
A person whose gender differs from the sex they were assigned at birth. This term includes transgender women, transgender men, and non-binary individuals. If you aren't sure what a person's pronouns are, just ask!
Queer is an umbrella term for any gender or sexuality that is a minority. Essentially, it entails anything that is not straight or cisgender.
Anybody who is questioning is exploring their identity (gender, sexuality, or otherwise). They may not be fully comfortable in aligning themselves with one identity yet and will instead tell people that they are questioning.
Intersex is an umbrella term that refers to any ambiguousness that might be present in an individual's sexual biology. Those who are intersex do not have physical characteristics that fit perfectly into the categories of "boy" or "girl". This can include anything from chromosomes to genitals.
Asexual people are individuals who experience little to no sexual attraction for any gender. Asexuality is a broad category with many sub-groups. Asexuality is not celibacy, and plenty of asexual individuals still experience romantic attraction.
The LGBTQ+ community is a vast community with entire spectrums of identity which celebrate the human condition.