Gender and Sex, What's the Difference?

The difference in the words "gender" and "sex" is tremendous. When you refer to the word sex you generally caount it as what you were at birth (male, female), while gender IMHO, can be interpreted as how you act. If your a man and want to be a woman and act and dress like a woman or just claim to be your gender could infact be taken as female even though your a male. Do you get what i'm saying? If I wanted to be a girl I could call myself a girl and in all reality my sex wouldn't change but my gender for se could. Idk, those are just my thoughts, i'd love to know yours, let me know whatcha' think.