An Adam's apple is an anatomical structure which appears primarily in males. Because it develops during puberty, biologists classify it as a secondary sexual characteristic. The size of an Adam's apple can vary widely; some are extremely prominent, while others are less pronounced, and they even appear enlarged on some women as well as men. The primary purpose of the structure is to protect the delicate voicebox from injuries.
The official name for the structure is the prominentia laryngea. It is composed of cartilage which surrounds the larynx. As the larynx grows during puberty, the cartilage enlarges to accommodate it, creating a bump. Over time, an Adam's apple will grow quite solid; young people have soft, flexible cartilage, while older people have more bony protrusions. In some cases, people find it aesthetically undesirable, and they undergo a surgery known as a trachea shave, in which the structure is greatly reduced in size.
There are two conflicting explanations for the origin of the common name for the thyroid cartilage in men. Some people say that it is a reference to the fact that it looks like of like a chunk of apple stuck in the throat, so the name is linked with Adam's consumption of the Fruit of the Tree of Knowledge. Others say that it is a result of mistranslation from the Hebrew tappuach ha adam, which just means “male bump.” The second explanation is probably more likely; many such malapropisms from Greek, Hebrew, and Latin can be found in modern English.
As people with Adam's apples know, the growth of this structure is associated with a change in voice. As men mature, their voices deepen and sometimes become more rough as well. The period in which the Adam's apple develops is often marked by wild fluctuations in the male voice, as the vocal cords settle into their new size. Men with especially large protrusions can also see and feel the movement of the cartilage as they swallow and speak.
Technically, everyone has a prominentia laryngea, even if it's not visible. You can feel this cartilage in your throat by finding your voicebox; start by humming and feeling your throat to feel the source of the vibration. When you find the larynx, you have also found your thyroid cartilage, since it encircles your voicebox. If you are younger, this anatomical structure probably feels soft and almost spongy, and you may be able to change the pitch and timber of your voice by gently manipulating your thyroid cartilage.