Gender describes a set of characteristics that make something masculine or feminine. They include your biological sex, the way you look, and the way you act. Someone may be born of male sex but strongly identify as female, so their gender is female. Such people are usually called transgender. The woman in the photo is biologically female, looks feminine and is undertaking the traditionally female gender role of cleaning the bathroom.
Someone is heterosexual, or straight if they are are attracted to the opposite sex. Most people are heterosexual. The opposite of heterosexual is homosexual.
Someone is homosexual if they are attracted to people that are the same sex as them. Homosexual men are often called gay, and homosexual women are often called lesbians. The opposite of homosexual is heterosexual.