I'm still confused. Intersex people do obviously exist, but if anything, aren't they the exception that proves the rule? When hormones 'misfire' during fetal development? Otherwise you've either got XX chromosomes or XY chromosomes encrypted in your DNA, something which every single cell in your body carries. Biological sex looks to me to be the most black-and-white thing a person can experience, short of alive or dead. And if gender is not based on biological sex, then what defines a guy or girl? If it's not biological, and it's not societal norms (if a guy can do girl things and vice-versa) then what is it? If I say, 'I'm an Oglorp' it doesn't do us much good if we don't have a dictionary with a definition of 'Oglorp.'
Soo, I'm going to chime in here because you sound a lot like me about a year ago, RTR. But huge disclaimer time first, because I really really don't want to offend anyone by accident and this is sometimes a sensitive topic.
Disclaimer: Basically all I know comes from the Internet, so you know how reliable
that is. I've never really dealt with gender identity problems myself (the closest I've come is being
slightly less girly than other girls and not wanting to wear dresses/high heels). I just decided to read up on some of this stuff when it was briefly mentioned in my psychology class last year. I may say stuff that's absolutely and entirely wrong. I didn't mean to. Please, please correct me if I do, and know that it was an accident. Maybe I'm being overly cautious here, but I really really really don't want to make anyone upset.
Phew. Okay, disclaimer over. I'm going to put stuff in spoilers because it looks like I'm going to be talking for a while. I think this stuff can be mostly explained in three categories: sex, gender, and expression. I'm going to not talk about sexual/romantic orientation (who is attracted to whom) because that's a whole 'nother mess of complicated, and I'd be here all night if I tried to do both at the same time.
So first we have the idea of sex.
Sex is what's between your legs/encoded into your DNA/whatever. It usually goes XX = female, XY = male, but like everything in life, it's really more complicated. I don't know how much biology anyone here knows, but the Y chromosome is actually pretty tiny: it doesn't have a whole lot of vital information coded into it. And the XY configuration proves that you only really need one X chromosome to function. So embryos with chromosomal abnormalities in the sex chromosomes (as opposed to the other 22 pairs of chromosomes we have) are much more likely to survive and be born and stuff (the only other chromosome pair that can take that much abuse and still result in a viable embryo is 21, and that results in Down Syndrome). People can be born, therefore, with chromosome configurations like any of these: X-, XXY, XYY, XXX, etc. and are generally mentally capable human beings. Usually in these kinds of situations a person is called "male" if they have a Y chromosome and "female" if they don't. But these people may have physical differences which don't match up with what "male" and "female" bodies generally look like (male and female referring to typical sex characteristics of XY and XX individuals, respectively). There's also the further complication of developmental abnormalities that aren't genetic but also make a person's body different from the aforementioned "male" or "female" ones. And yeah, with all that can come hormonal problems and also body image issues and a whole host of other things. But that's mostly sex, not gender.
That was probably the least complicated of the bunch of these. Next is gender.
So gender is kind of a hard concept to wrap your head around because it's so abstract. It's also probably the one I'm going to mess up explaining the most, but here goes anyway. Gender is kind of how you mentally categorize yourself. I would probably go to agree with Haiz in that it's largely if not entirely a social construct: there are some societies that have three or four different genders. Ours has two: boy and girl. You're put into one of the two based on your sex at birth, and that's where most people stay. People like me are fine with the category we've been assigned, and it fits. But for some people, it doesn't feel like they fit in the category they've been assigned. Maybe they look at the other category and go "that seems so much more like me than this category". Or maybe they kind of pick and choose ideas from each category and feel most comfortable somewhere in the middle of the two categories. Some people feel like there's a third category out there for them somewhere. And some people are just confused by all these categories we seem so preoccupied with and prefer to do without them entirely. Mind you, the "categories" aren't necessarily stereotypical things like girls liking the color pink and guys being aggressive. They're more nebulous and subtle than that. It's a really hard thing to wrap your mind around, and I don't even get it completely, but that's really the best I can do to try and explain it. It's also a really personal thing, and so my ideas of what gender is may not match up with other people's ideas. But for what it's worth, this is my perception of what gender is.
Phew. I don't know if I explained that well at all. Nevertheless, on to gender expression!
In short, this is how you act. Our society has a lot of preconceived notions of what are masculine and feminine behaviors, and so your particular combination of behaviors places you somewhere on a masculine-feminine scale. This can include how you dress, what you talk about, how you carry yourself, and basically everything else that you do. I would argue that there are gender neutral behaviors too (mostly the kinds of things that are necessary for survival), but like anything there are nuances to even those that may give off masculine or feminine vibes. It's also probably the one that the most people fall in the middle of the spectrum for. For example: I am biologically female and consider myself a girl, and I mostly do feminine stuff but I also do some masculine stuff. And so I probably fall somewhere on the girly-but-not-100%-girly end of the spectrum.
All of these things are a part of gender identity, and they can be mixed and matched in all sorts of different ways depending on the person. It's very confusing and very complicated. but like ParanormalAndroid said, the most important thing is to find where you fit into all of it if you haven't already, and respect where other people find themselves. I'd like to type even more, but I'm maybe rambling at this point, and I have to go to some sort of event, so I'm going to awkwardly end my really long post now. Again, if I was wrong or hurtful or offensive for any reason, someone please please tell me so I can apologize profusely and fix it!!