They’re the actual terms for the people, basically. I’m… neither, I suppose–I’m nonbinary. But no gay person has ever used “mxm”, for example, to describe anything in their lives. Whereas WLW and MLM are actual helpful descriptors that human beings use. It started on social media as a way of identification–hey, I’m a girl who likes girls, are you, too?–and is now fairly well known in any LGBT circles.
Feminine experience with masculinity… that just sounds like a woman. Unless she’s trans or bi or something, she’s not LGBT for that inherently. You could say she’s GNC (gender non-conforming), that is to say, that she does not conform to gender roles. This generally refers to presentation, so here’s a nice picture of what I get when I Google for that:
You can see that the people in this image are not at all trying to fit into any binary of expression. Doesn’t mean that they’re LGBT inherently, but this is of course more common in the community for, oh God, so many reasons.
Quick glossary (since I don’t trust almost anything for a 101): LGBT stands for Lesbian, Gay, Bisexual, Transgender. Lesbian meaning a woman who likes exclusively women, gay meaning a man who likes exclusively men, bisexual for people who like two or more genders (this is me!), and transgender for people who are not the gender they were assigned at birth (also me!).
Other letters have been toyed with for adding to the acronym, for various reasons, but this is the original and most commonly used. You may often see a + sign or Q, also for various reasons–I think the + is best at saying that no combination of letters will include everyone, and that’s okay. If you’re Not Cis or Feel A Certain Way Towards The Same Gender, you’re probably in here.
(Nonbinary = not strictly a woman or a man. Can also mean a lot of things–agender (no gender), bigender (two genders), genderfluid (gender changes), etc.)
And then for trans I may as well add more. Essentially, here’s the process–a baby is born. The doctor labels it either girl or boy based on external genitalia. The baby grows up, raised a certain way based on this label. Trans people are people who get older and eventually go “wait crap this isn’t really my gender” for various reasons, and cis people are people who… do not do that. Trans people may transition in one way or another (clothes, hormones, surgery) to look different… or they might not. That bit is complicated and very personal and the #1 rule is, as always in society, “do not ask about someone’s genitals.” This is the same for cis and trans people, really; just don’t ask about what someone’s bits look like.
It’s alright to have questions! I’m glad you’re asking. Feel free to ask more.