Maturity. Is she responsible? Can I trust her? Is she moody? Does she bitch/complain? Is she helpless or self reliant? Does she work toward bettering herself or sit around with the status quo? Does she want a committed relationship or ride the cock carousel? Is she good with kids or not?
In person I refer to someone as a girl if she is nice and cute/pretty. I refer to others as women if I don't find them pretty, or if I don't care for their personalities enough to be friends. Online, I just use the terms interchangeably and there is no difference.