A useful explanation can be found below. It is taken from the Sylvia Rivera Law Project
You should also google and find out more information should you need it.
What is "Gender Identity"?
• “Gender identity” refers to how people see and identify themselves; for example, some people identify as female; some people identify as male; some people as a combination of genders; as a gender other than male or female; or as no gender. For example, transgender women identify as women but could have been classified as males when they were born. Transgender men identify as men but could have been classified female when they were born. Everyone has a gender identity.
• “Gender Expression” refers to how people express their gender identity. Everyone expresses their gender identity in different ways: for example, in the way they dress, the length of their hair, the way they act or speak and in their choice of whether or not to wear make-up.
WHAT DOES “TRANSGENDER” AND “GENDER NONCONFORMING” MEAN?
• “Transgender” is a general term used to describe people whose gender identity differs from the sex they were assigned at birth.
• “Gender nonconforming” refers to people who do not follow other people’s ideas or stereotypes about how they should look or act based on the female or male sex they were assigned at birth.
• “Transgender” and “Gender nonconforming” are umbrella terms that often encompass other terms such as transsexual, cross dresser, gender queer, femme queen, A.G., Two Spirit, and many more. It is important to refer to people with the term they prefer.
• “Gender Questioning” People who are questioning their gender identity might be wondering whether they identify as a man, woman or another gender. They might also be experimenting with different genders.