The difference between Gender and Sex is one of the founding principles of modern understandings of gender. This principle states that gender identity - the internal psychological experience of gender - is separate from, and not necessary aligned with, the physical sex characteristics of your body. This can be difficult to understand for those whose gender identity matches their assigned sex (cisgender) but can result in discomfort and dysphoria for those whose internal experience of gender does not match how they are viewed by others and society (transgender).
According to the World Health Organisation, gender is considered to be a social construct which reflects the gender roles of a particular society. Because of this, gender can vary between cultures and over time. In Western culture, gender is seen as a binary choice between male and female, although there is increasing awareness of transgender and non-binary identities. Other cultures may already have three or more genders, such as recognition of Two-Spirit people in many Native American societies, or Hijra in India.