When I was in high school and younger, we were "girls" and that was that. When I went to college on the East coast, all of a sudden we were "women". There was a strong push to call all the female students women, and the word girl was even considered offensive. People joked about it, but it was also very pervasive. I and all of the females I knew were now to be called women. I modified my language accordingly.
After college, when I entered the real world, the word girls came back. It surprised me at first, given how essential it had seemed at college to refer to females out of high school as women. Many of the women I met and worked with referred to their friends as girls, and I found myself relaxing with the term.
Later I came to think that all of the fervor in college probably had to do with "coming of age". As college females tried to find their identity and embrace a grown-up status, they considered the word girl to be backtracking. It was a name for children, and we were trying to grow beyond childhood. Yet at the same time, we really weren't adults yet. We were almost all full-time students, almost all just out of high school. We were all at different levels of maturity, trying to figure out what we wanted to be when we grew up, even while pretending to already be grownup.
College females, I thought, insisted that they be called women because they weren't secure in their identity as such. It's a little bit like the way I used to be desperate to look unique in my clothing choices when I was younger, for example. I fought to proclaim to the world that I was different and interesting. Now I'm more secure -- I know I'm different and interesting, and I don't feel a need to shout it out with my clothing or hair or whatnot.
Now that I am more settled and more comfortable in my womanhood, I no longer am bothered by the word girl. Well, most of the time. Informally, I use it all the time. With friends, I'll use the words girls, women, or even ladies when being playful. In fact, the word women almost seems too formal and pretentious for casual use.
I think of myself as a girl most of the time, I suppose. After all I'm the JAVAGRL, and a girl geek, and I like some girly sorts of things. But at the same time I am also a woman -- strong, confident, self-sufficient. But at the same time I'll talk about Girl Power and whatnot. Maybe the difference in the terms is whether I'm feeling playful or serious. I'm not sure.
Professionally, it still seems that "woman" is a more appropriate word. This is particularly true with women I do not know well, or when referring to them with others. I feel uncomfortable to be called a girl by colleagues, even though I'll occasionally refer to myself as one when joking around.
I remember at my first job, I shared a cubicle with a woman. She and I had the same first name. A colleague of ours would stroll in often and say "Hi girls." We three (all about the same age) shared a friendly relationship, and joked around all the time, but it still felt weird. We bugged him about it until he stopped. He asked us, "Well what should I call you?" We suggested "guys is fine, women, folks, people, ladies, or maybe just use our name -- we even have the same one after all!" (I don't remember if he switched to another term, or if he just started saying something different when he approached us. It was awhile ago.)
It's an interesting question, and language is a funny thing. Part of the problem is that there is no female version of "guy". Although we embrace it in the plural, and a group of women can be called guys, it still isn't quite right to call one woman a guy. So in casual conversation I'll call women "girls" -- but to make up for it, sometimes I'll call men "boys". I figure that evens things out a bit.