The ear is part of a sensory system found in most carbon-based lifeforms, typically, but not always, consisting of a fleshy outer appendage usully called the outer ear, as well as a group of canals and vibration-sensitive membranes and small bones located inside the skull.
Ears function by recieving vibrations in the ambient atmosphere and translating them into electrical impulses sent to the brain, which allows a humanoid to 'hear' sounds around them.
Most humanoid species' ears are located on or around the outer areas of the skull, and the sensitivity and range of sounds audible varies greatly among species.
Human ears are typically mostly round, fleshy appendages, while Vulcan ears are more typically pointed at the top. These features made Vulcans the frequent subjects of ridicule by humans, although the stoic Vulcans were confounded by human "fascination" with their ears.
The Ferengi have large, rounded ears, the rims of which they refer to as "lobes". Physically, the lobes are a highly sensitive area that produce feelings of pleasure and contentment when stroked, as in the technique called oo-mox. However, the lobes are also very sensitive to pain. (DS9: "Family Business")
Many species, such as the Gorn have no external appendage or visible 'ear' but can hear as well as, and in some cases far better than, species with external auditory organs.
The sensitivity and hearing ranges for species vary widely, even on the same planet. Vulcan hearing is known to be more acute than that of most Humans.
The state of being unable to hear, whether by genetic makeup, congenital defect, or external damage to a being's hearing is known as deafness. Some societies consider deafness a weakness or handicap, while others have no social stigma attatched to it. Many well-known beings throughout history have been deaf, including eighteenth century Earth musician and composer Ludwig von Beethoven, and twenty-fourth century mediator Riva (TNG: "Loud as a Whisper").