A week ago in her blog, Cheryl posted an excellent brief Gender 101 entry, covering a variety of topics: biological sex, gendered behavior, gender identity, legal gender, "trans" and related terms, and so on.
The same day, in the Convention Reporter blog, Cheryl posted some background info and further reading for the Future of Gender panel from this year's WorldCon. Sex changes in nature, hormone pollution, numbers of intersex and trans people, etc.
Both entries provide useful info and are well worth reading.
See also Human Rights and Gender Identity, an issue paper from the Council of Europe Commissioner for Human Rights.