It refers to a person primarily emotionally, physically, and/or sexually attracted to members of the same sex/gender. It "also refers to a person's sense of identity based on those attractions, related behaviors, and membership in a community of others who share those attractions."
More about Being Gay/Lesbian
Gay - The adjective used to describe people whose enduring physical, romantic, and/ or emotional attractions are to people of the same sex (e.g., gay man, gay people).
Lesbian - A woman whose enduring physical, romantic, and/or emotional attraction is to other women. Some lesbians may prefer to identify as gay (adj.) or as gay women.
Homophobia – noun : an umbrella term for a range of negative attitudes (e.g., fear, anger, intolerance, resentment, erasure, or discomfort) that one may have towards members of LGBTQ community. The term can also connote a fear, disgust, or dislike of being perceived as LGBTQ. Homophobic – adj. : a word used to describe an individual who harbors some elements of this range of negative attitudes towards gay people.
May be experienced inwardly by someone who identifes as queer (internalized homophobia).
Avoid identifying gay people as "homosexuals" - an outdated term considered derogatory and offensive to many lesbian and gay people. The term - “homosexual” is considered stigmatizing (particularly as a noun) due to its history as a category of mental illness, and is discouraged for common use (use gay or lesbian instead). Until 1973 “Homosexuality” was classifed as a mental disorder in the DSM Diagnostic and Statistical Manual of Mental Disorders. This is just one of the reasons that there are such heavy negative and clinical connotations with this term.
Adapted from A Guide to Gender (2nd Edition): The Social Justice Advocate's Handbook and GLAAD Media Reference Guide, 10th edition.