I am biologically female, not intersex (someone who is intersex has both male and female sexual characteristics). i.e. my sex is female. I look female (although I am often mistaken for male until people see the size of my chest because I dress very unisex/male).
My gender is how I feel - what I feel like I am. I do not feel female (and never have despite trying to be female for 48 years) and yet I do not feel male either and so I am agender not transgender. I do not want to transition to male, either. I just want my breasts removed (not that I can afford it and this is almost impossible on the NHS) so that I look more like my gender. For myself.
My sexuality is asexual, autochorissexual to be precise.
My preferred pronouns are they/them/their. I answer to she/her/hers or he/him/his as well, especially from people face to face who do not know me. Asking a person's pronoun preferences f2f in the UK at the moment is VERY rare.
I hope this suffices as a very brief primer (with focus on myself) to these three distinct categories. Sex, gender and sexuality are three very different things.
P.S. I am writing this with a migraine (non-ocular, mostly vestibular and painful) that started yesterday morning. I hope to remember to write a longer version of this post at some point but this is a brief summary. Written in reply to a reply to my earlier post.
I need some gender/sexuality icons but I suppose that my rainbowed-puffin will suffice for now.
201604031344BST: Edited to change "agenda" to "agender" in the quoted text and add a link to the LGBT symbols on wikipedia.
201604041902BST: Edited to add that I also look male because I have wide shoulders, long arms and legs (a male frame apart from my hips, I suppose) and, apart from the breasts, said hips and lack of penis, fit better into men's clothes than women's.