'Nonbinary' is an umbrella term used to describe people whose gender identity does not fall into either of the categories 'male' or 'female', i.e. people who find themselves outside of the gender binary. Another term with a similar meaning is 'genderqueer'. I use both of these words to describe myself. Although I was assigned a binary gender at birth and brought up accordingly, I don't feel that has much to do with who I really am.
I have switched to using the name Rae in most situations (a more gender-ambiguous variation on my given name). If you know me in person, please call me that!
I prefer the title Mx over Mr or Ms, and definitely never Miss or Mrs.
I use gender-neutral pronouns. In English, the most widely used is they. For example: The developer realised they had been sitting at their desk for far too long. They stood up and stretched their back.
In some other languages, the options for gender-neutral pronouns are less obvious. German has xier: Xier, dier jongliert. Xier konzentriert sich auf xiese Ringe. I don't know enough French to find out what gender-neutral pronouns are used by French-speakers. Swedish, meanwhile, now includes hen in the dictionary.
If you refer to me, please use gender-neutral pronouns to do so! I won't be angry if you forget or find it hard to change, but it will mean a lot to me if you remember.
I am generally happy to discuss gender stuff in person, say in a coffee or lunch break, but I try to avoid getting in internet arguments about it. :)