Vulva, the external structures of the female reproductive system includes the labia (singular, labium), the major externally visible portions of the vulva. The external genitals of the female include the vaginal opening, the clitoris, the labia minora, and the labia majora, collectively called the vulva.
Labia are folds of skin and mucous membranes. Labia cover and protect the opening to the female reproductive system (vagina and urethra) and the clitoris as well.
Thus there are two pairs of labia: Labia Majora and Labia Minora
(1) Labia Majora, the outer labia are larger, fattier and hair-covered folds of skins. Upon sexual stimulation the labia majora expands, redden, and spread away from the vaginal opening.
(2) Labia Minora are two small folds of skin concealed within the labia majora. The labia minora help to keep germs out. The clitoris, a small organ containing erectile tissue that is highly sensitive to stimulation, is partly enclosed by the labia minora. Upon sexual stimulation labia minora along with vaginal wall and clitoris become engorged with blood. The cleft between the labia minora contains the openings of the urethra and the vagina.