In the field of audiology, the external structures of the ear are most often referred to as the 'pinna'. While the term 'auricle' is still sometimes used, it is uncommon.
The pinna refers to the part of the ear that most of us think of when we refer to ears; the bit that sticks out from our head. It is a cartilaginous structure, and while there is certainly variation among human ears, many structures are common enough to have technical names. The outer rim is the helix, and the inner rim or fold is the antihelix. On most ears these to rims meet at their lower terminus on the ear lobe. The little cartilaginous protrusion that sticks out over the entrance to the ear canal is called the tragus. The little bump or joggle just before the antihelix meets the lobe is the antitragus. Between the tragus and the antitragus is a gap known as the intertragal notch. The central 'cup', adjoining the entrance to the ear canal, is the cavum concha, or simply the concha. The entrance to the ear canal is called the external auditory meatus. This is not a full list of terminology, but these are the major landmarks of the pinna. You may also have come across the term Darwin's tubercle, a thickening on the rim of the helix that is sometimes used in biology classes as an example of an autosomal dominant trait.
The pinna is shaped to funnel sound into the ear canal, and this aids in amplifying certain frequencies. The adult concha is shaped to hold a 'pocket' of air that has a natural resonant frequency of 5000 Hz.; this is paired with the shape of the ear canal, which has a resonant frequency of 2500 Hz. This has the effect of amplifying these frequencies at the eardrum by approximately 10 to 12 decibels; the middle ear, on the other hand, is optimized for amplifying sounds from about 100 Hz. to 2000 Hz. Together these structures amplify the frequencies from 100-5000 Hz., which are the sounds most important for understanding human speech.
The shape of the pinna also aids in helping us localize the direction sounds come from. Having one ear on each side allows us to easily determine which side a sound is coming from, but in order to judge front-to-back localization, we would have to add another ear. The Pinnae help us get around this by amplifying sounds that are at eye level and directly in front of the listener, and attenuating sounds that come from behind or from above or below.
And finally, the pinna, and particularly the tragus, also helps to protect us from foreign objects entering the ear canal, and the intertragal notch allows gravity to remove sweat, dust, dirt, bugs, and crumbles of earwax from the concha with a minimum of fuss.
The primary reference for this writeup was Audiology: The Fundamentals by Fred H. Bess and Larry E. Humes. This was supplemented by my class notes.