Gender: The constellation of socially constructed behaviors, roles, characteristics, appropriate activities, propensities, etc. that correspond to "masculine" and "feminine."
Sex: The (somewhat artificial) binary of physical and biological characteristics thought to distinguish male from female.
Gender Identity: The psychological aspect of gender - the way one identifies oneself, which may or may not match the gender and/or sex assignment at birth.
Cisgender: A person is cisgendered if their gender identity matches the gender they were assigned at birth based on their physical sex.
Transgender: A person is transgendered if their gender identity does not match the gender they were assigned at birth based on their physical sex.
Cissexual: A person is cissexual if their gender identity matches their sex assignment at birth.
Transsexual: A person is transsexual if their gender identity does not match their sex assignment at birth.
Genderqueer: A person is genderqueer if they reject the gender binary by blending aspects of both genders or forming a third gender identity or simply trying to live without being defined in terms of gender.
Intersex: A person is intersex if they don't fit neatly into either the "male" or "female" categories. This may be due to ambiguous genitalia or a mix of typically male and female reproductive and/or sexual anatomy. Traditionally, intersex individuals have been surgically altered to fit into the gender categorization that's assigned to them. However, intersex is slowly being recognized as a normal condition that has been problematized by the rigid sex binary required by our cultural attitudes and gender binary.
*Note: "hermaphrodite" is generally a pejorative term used to describe intersex
Social Construction: Any contingent phenomenon that is created by a society. Social constructs exist only because the members of a society implicitly agree to behave as if they do. Generally speaking, there are conventions around social constructs that guide our behavior regarding them. The most common example used to illustrate this is money. Paper and gold money would be worthless if it weren’t for our practices and conventions, but because we all agree to invest money with value, it is valuable. Saying that money is a social construct doesn't imply that it doesn't exist or that it's not real. It's very real and exists as a social construction; it's just not a "natural fact" about the world independent of human activity.