Labium (plural labia) is a Latin-derived term meaning "Lip" and often specifically refers to parts of the vulva.
Anatomy and Medical:
The labia majora are lip-like structures comprised mostly of skin and adipose tissue, which extend on either side of the vulva to form the cleft of venus through the middle. After puberty, it is typically covered with pubic hair, which some individuals choose to remove by shaving, waxing, or laser methods. When standing or with the legs together, they usually entirely or partially cover the other parts of the vulva. Protection is the main function.
The labia minora (obsolete: nymphae) are two soft folds of skin between the labia majora and to either side of the opening of the vagina. The clitoris is anterior to the vulva where the labia minora meet superiorly. The visible tip of the clitoris, the clitoral glans, is entirely or partially covered by a "hood" of tissue (the clitoral hood).
The coloration, size and general appearance of the labia can vary extensively from woman to woman. In some women the labia minora are almost non-existent, and in others they can be fleshy and protuberant. Usually, but not always, they are symmetrical. Some differences are purely personal, while others may be genetically linked; a striking example of the latter is the elongated labia minora of the Khoisan peoples, whose "khoikhoi aprons" can hang down up to four inches past their labia majora when they are standing.
During sexual arousal, the labia minora become engorged with blood, typically swelling slightly and darkening or reddening in color.
abridged version derived from: http://www.wikipedia.org