Gender?

Merc
To begin: I'm cis so if I cause any offense with this post, please let me know so I can reword it! I've been thinking about gender and its fluidity for a while now and I was curious to others' opinion on it.
I understand sex and gender are different in that sex is your physical attributes and gender can differ from that. But does gender align itself with stereotypes? Otherwise how else would one feel more feminine or masculine? How would one know what is to feel like a woman if there were no gender roles/stereotypes in place?
For trans people: how is being trans different that being a girl who feels more masculine or vice versa?