There are some words that have so much power, both to make people feel good about themselves and to make them feel terrible, that to use them ethically means to make them complicated, and resist simple definitions. Gender is one of those words.
As a sex educator I can offer a working definition of gender which is what you'll find below. As a human, and someone who cares about other people and supports people's right to define their own experience, I need to point out that my definition is no the best one or the right one. Hopefully what it is, is a place to start thinking about gender for yourself.
People (including professionals who should know better) often confuse the words sex and gender. Usually when we use the word sex to describe a person we are talking about their physical body, and their genetic make up. Sex, in this context describes genitals and chromosomes. Gender is a term that is used to describe the social and cultural meaning we ascribe to the bodies we have. Sex and gender may be related but they aren't the same thing.
Gender isn't biological. It's something we experience in our bodies, but it's also something that is experienced outside our bodies. Gender can refer to social expectations and stereotypes about being masculine or feminine, it can refer to our own internal experience of our bodies and these social expectations, and gender also refers to the conscious and unconscious ways we express how we feel in the world.
Another important thing to know about gender is that it isn't an either/or choice. Most people still think that there are only options for gender, male or female, masculine or feminine. This isn't accurate. There are many different ways that gender is expressed, and we know that while all children explore gender roles, for some people gender fixed throughout their lives, it is fluid and changes.
Social expectations and stereotypes define gender for us in countless ways. Some of the more obvious ones are statements and beliefs about children that insist that "boys do such and such" and "girls like this or that" where the specifics change based on time and geography, but they are always strict rules and the penalty for a boy who likes "girl things" or a girl who likes "boy things" can be severe.
For adults, social expectations and stereotypes about gender result in the beliefs like "men are from mars and women are from venus". Gender is defined not only by what you do or what your preferences are but by how you communicate and interact with the world. Social expectations and stereotypes make up a lot of how gender is understood in our world, even though they often have little connection to how we feel about gender.
Our internal experience of gender is something different. This refers to the way we feel, think, and experience ourselves as somewhere on the continua of masculinities and femininities. Social expectations pressure all of us into thinking of this as an either/or choice. Either you are a man and therefore you act in masculine ways or you are a woman and you act in feminine ways. The truth is that while many, probably most, people have an internal experience of masculinity and femininity, few if any of us feel 100% one or the other all the time. And many people experience gender as something different than either masculine or feminine, in fact they feel themselves to be clearly in between or outside of these choices altogether. So saying that gender is only about being masculine or feminine isn't really telling the whole story, since most people feel both ways at least some o the time, and some people experience gender as a third thing.
Gender can also be understood in terms of how we present or express ourselves in the world. It might be how we talk or move around or dress. It might be about what kinds of people we spend time with or how we structure our relationships. This is often referred to as gender expression, because it's the part that the world sees or experiences from us. The way we feel inside about gender may or may not match the ways we present our gender to the outside world.
Just as a medical professional assigned us a sex when we were born, other people in the world treat us as if they know what our gender is. But because gender describes an internal experience, it is something that may not match the way we look on the outside, and others often can't know our gender identity without asking. This doesn't stop most of the world from making assumptions based on the way a person dresses or talks or behaves.
Despite societies efforts to force us into one of two boxes, most people challenge gender expectations or roles in some way. Some of us do this more obviously and explicitly than others, but none of us can fit the expectations of "real men" or "real women" all of the time. And to make matters more complicated, the social expectations are always being challenged and changing. So what makes a "real man" in the 1970s may be very different from what makes a real man in the 2010s, even in the same community.
The only constant when it comes to gender is that we are supposed to follow someone else's rules, and we should "fit in" meaning that our gender expression should match people's expectations of our gender based on what we look and sound like, and how we move through the world.
Of course some of us are better at fitting in than others, or seeming like we are following the rules. It is usually the people who visibly don't fit expectations about gender that are most punished in any culture. Sometimes punishment means being left out, other times it means being the victim of violence.
There are many different terms that people use to describe their gender or gender identity. And this language is forever evolving. So there are terms that some people use and that others don't, or that some people think shouldn't be used to describe gender, but saved for some other identity description. What is most important is that people have the right to identify themselves, and not be forced to take a label to fit someone else's expectations. Here are some examples of words people use to describe their gender: man, woman, transgender, transgender man, transgender woman,cisgender, cis-man, cis-woman, transsexual, genderqueer, gender variant, Two-Spirit, Third Gender. Androgyny is sometimes used by people as a gender identity, although for others the term describes their experience of total gender ambiguity, which makes them feel as if they don't think gender itself is a useful frame for their lives. There are so many different definitions for each of these terms that it's hard to know where to suggest you begin. In 2011 the National Lesbian and Gay Task Force issued a report Injustice At Every Turn which includes a carefully developed glossary in the back.
This is a time of active debate about these ideas and terms for gender. While it might seem easier if we could all agree on one, five, or twelve definitions, the result would never be as useful or good.