The opposite of cis. You feel the gender assigned to you at birth is incorrect and being called that gender (afab called a pretty girl for example) is upsetting to you. Seeing your body not conform to your gender is distressing and you have dysmorphia over it. Transgender is often thought of as being the literal opposite of your assigned gender (female to male, male to female) but many queer and nonbinary people also identify as trans (but not all of them).
Some trans people never wish to have gender affirming surgery or even medicine. Some do. Despite hysteria most can not access these in English speaking countries under 16. Right now they are heavily targeted by bigoted idiots. Trans people often do chose to (but not always) use a new gender affirming name and pronouns. If you’re unsure just ask said person! And never, I mean fucking never, ask or accuse a trans person about their genitals. Unless you are sleeping with them or their doctor this is not your business.
Many Trans people do what is called a social transition. In addition to names and pronouns they may style themselves and wear gender affirming clothing. However if a trans person doesn’t, this does not make them less trans. There is no one way to be trans correctly. Do what feels right for you! And as long as you feel safe in doing so, stand up for your rights and name/pronouns. Trans is extremely special: they are the only gender in the LGBTQ letters (depending how you count q).
Genders included in Trans (some are not limited to):
Female to Male Trans (FMT)
Male to Female trans (MTF)
Nonbinary Transgender
Transfemme
Transmale
Transexual
Abinary (a lack of binary, ie female or male)
Ambigender/Bigender/Plurigender/Multigender/Dual Gender
Androgyne/Androgynous/Aporagender