Gender is the range of characteristics pertaining to, and differentiating between, masculinity and femininity. Depending on the context, these characteristics may include biological sex (i.e., the state of being male, female, or an intersex variation), sex-based social structures (i.e., gender roles), or gender identity. People who do not identify as men or women or with masculine or feminine gender pronouns are often grouped under the umbrella terms non-binary or genderqueer. Some cultures have specific gender roles that are distinct from "man" and "woman," such as the hijras of South Asia. These are often referred to as third genders.
Gender studies is a field for interdisciplinary study devoted to gender identity and gendered representation as central categories of analysis. This field includes women's studies (concerning women, feminism, gender, and politics), men's studies and queer studies. Sometimes, gender studies is offered together with study of sexuality.
Gender is a huge hot-button issue for lots of people who feel strongly about it, I am not interested in triggering those strong feelings.
Sue Gardner, as quoted in "Define Gender Gap? Look Up Wikipedia’s Contributor List" by Noam Cohenjan, New York Times, (30 January 2011)