Understanding the difference between Gender and Sex could be one of the most important steps to understanding transgenderism.  The basic idea is that gender refers to your identity, the mental, while sex is your physical, biological, or legal classification.  For most people, the difference is moot because their gender identity matches their assigned sex, which can make it harder to understand that there is a difference.  For people who are transgendered, however, their gender identity does not match their biological sex.