In everyday conversation, English speakers usually use the words “sex” and “gender” interchangeably. Many languages such as German, Mandarin, and Persian don't even have separate words for the two. But in an LGBT context, it can be helpful to understand and approach sex and gender as separate concepts.
Sex is a biological category that refers to attributes in humans and animals related to reproduction, specifically their reproductive system and secondary sexual characteristics. In humans and other mammals, individuals are either male (producing small, mobile gametes called sperm) or female (producing large, immobile gametes called eggs or ova).
Gender, by contrast, is a human construct, a diverse set of practices and norms that emphasize and communicate social and cultural distinctions between the sexes. The term “gender role” refers to societal expectations about how an individual should dress, speak, and behave based on that person's biological (or perceived) sex. These roles typically center around femininity (characteristics generally associated with females) or masculinity (characteristics generally associated with males).
Gender identity refers to a person's inner sense of their own place within the cultural practice of gender; that is, where and how they fit — or don't fit — their society's categories and norms. A cisgender person is someone whose gender identity aligns (or at least isn't at odds) with societal expectations for their biological sex. A transgender person is someone whose gender identity does not correspond with cultural expectations for their biological sex. Concepts of gender have varied dramatically throughout history and across different parts of the world. For example, before World War I, Americans widely regarded pink as a bold, masculine color while they considered blue to be calm and passive, and therefore feminine.[1] The genders available to an individual in a given society are not necessarily limited to two. One could imagine that there are as many genders as there are people in the world since everyone expresses themselves differently.
The concept of sexual orientation frames human attractions in terms of biological (or perceived) sex. This is because the very first LGBT activists developed the concept of sexual orientation as a tool to fight against the criminalization of same-sex behavior. As a separate human rights issue, they advocated for freedom of gender expression, regardless of orientation. You can read more about this history in our Bi101: Sexual Orientation.