Content Warning: this page includes mentions of coercive gender/sex assignment.
Things aren't always as they seem...
A person's gender identity is the innate way in which they identify as male, or female, a mixture of both, neither or somewhere in between any of these points. It is very personal and entirely based on what is in the brain. A person's gender assigned at birth is the gender they've been brought up as, usually based on the appearance of their genitals when they were born. In the case of some intersex people, this can be even more medically determined than it is for most people. Although most people's gender identity lines up with their gender assigned at birth, trans people don't find this to be the case. A trans person should always be referred to as the gender they identify as (unless they ask otherwise) - this is their gender.
However, not everyone expresses themselves in ways traditionally associated with their gender identity. Cis and trans people alike can transgress these traditional boundaries with more creative forms of gender expression. This doesn't make their gender identity any less valid. All in all, a person's appearance doesn't necessarily tell you anything about their gender. Although it can be tempting to make assumptions based on appearance - be prepared that you might be wrong!