Very broadly defined as a "study of what is understood as masculine and/or feminine and/or queer behavior in any given context, community, society, or field of study," (Jule, 2014) such as in our case, - literature, - this theory looks primarily at the role of gender, its intersection (such as that with the sex) and role of people of a particular gender in a given text, in particular of gender minorities (for example, women, transgender people, etc.)

It should be noted that some literature focuses specifically on the issues of gender, sex, and gender identity- some examples include the young adult novel Zenobia July and the graphic memoir Gender Queer. 

We can use gender theory to examine how these works' protagonists' views on gender evolve through the book, how they interact with the society, or how their core identity changes (ex., Gender Queer)

On the other hand, we can still analyze "non-gender focused" literature through the lenses of gender theory and see how gender, sex and gender identity interacts with the social arenas of economics, social position and relationships.

While often used interchangeably, Sex and Gender mean different things. Planned Parenthood defined Sex as "a label — male or female — that you’re assigned by a doctor at birth based on the genitals you’re born with and the chromosomes you have. It goes on your birth certificate." 

On the other hand, Gender is defined as " a social and legal status, and set of expectations from society, about behaviors, characteristics, and thoughts. (...) But instead of being about body parts, it’s more about how you’re expected to act, because of your sex." 

On the other hand, some people might be born with features of both sexes, thus making them intersex. "These characteristics may be chromosomal, hormonal and/or anatomical, and may be present to differing degrees."