The clitoris is both an external and internal sexual organ, and is thought to be the part of the body that is most densely packed with nerve endings, and is sometimes referred to (unscientifically, since there's no way to prove this) as the only part of the body whose sole purpose is pleasure. While it isn't always the case, for many people clitoral stimulation leads to orgasm.
The clitoris offers a fascinating example of how even something like anatomy -- which we might think of as objective and not open to debate -- can be political and hotly contested. Even today, if you look up "clitoris" in various anatomy books you may find radically different information from one text to another -- and many texts include significant factual errors.
Much More Than a Nub of Skin
What exactly the clitoris is, where it begins and ends, and how important it is to sexual stimulation and pleasure, have been points of contention for centuries. There have been times when more detailed information was put forth in medical texts, only to disappear a generation later. The Australian anatomist and researcher Helen O'Connell, who has been studying the clitoris since the mid 1990s, documented this in a seminal paper in 2005 that set the record straight about the clitoris, at least for now. Conventional anatomical wisdom suggested that the clitoris was a small external part of sexual anatomy. Through dissection as well as ultrasound and other imaging techniques, O'Connell and her colleagues documented that the clitoris is much more than a small nub of sensitive skin, and that the clitoris extends beyond the part we can actually see.
The part of the clitoris that can be touched and seen can be found by following the labia up to the top of the vulva. The point at which they meet is a fold of skin called the clitoral hood, and the tip, or glans, of the clitoris is under the hood. This is the external part, and it's a part of the body that many people find very sensitive and highly pleasurable when stimulated.
The rest of the parts of the clitoris are collectively referred to as the internal clitoral body and make up a much larger part of the clitoris as a whole. From the head, the clitoris extends into the body. The clitoral shaft is about an inch long and while you can't see it, you may be able to feel it under your skin if you massage above the head (some people describe it as feeling like a small cord). There are two "legs" (called crura) which extend in kind of a wishbone shape behind the outer labia. On either side of the labia, there are also clitoral bulbs (called vestibular bulbs), which are about the size and shape of a pea pod extend further down the vulva usually right around the vaginal opening.
Thousands of Nerve Endings
The clitoris is incredibly sensitive, thanks to more than 8,000 nerve endings found in the tip of the clitoris, and the more than 15,000 nerve endings throughout the pelvic area that the clitoris interacts with. Many parts of the clitoris contain erectile tissue and become engorged with arousal and stimulation. Anatomists consider the clitoris to be analogous to the penis in that it develops from the same basic body parts in fetal development, and while they look different and have different names, they have they continue to have the same basic parts in adults (e.g. a head, a shaft, spongy tissue that becomes engorged with blood, etc…)
Federation of Feminist Women's Health Centers. A New View of a Woman's Body. Feminist Health Press, 1991.
Komisaruk, B.R., Beyer-Flores, C., Whipple, B. The Science of Orgasm. Baltimore: The Johns Hopkins University Press, 2006.
O’Connell, H.E., Sanjeevan, K.V., Hutson, J.M. "Anatomy of the Clitoris" The Journal of Urology Vol. 174. Iss. 14-1 (2005): 1189-95