lesbians are women who are attracted to other women. Gays are men who are attracted to other men. Bisexual is men or women who are attracted to either sex. Pansexual are people who are attracted to all genders. Ally's are people who support the queer people.
I think that the terms are used so closely because no one really understand the difference. The someones gender is who the think they are and who they think they should be. While sex is who you were physically born as.
Gender fluidity is someone who may not identify as male or female, but as both, neither, or as a blend. Its literally someone who is all genders and no genders all at the same time.