Otitis simply means an ear infection; most of the time when we talk about ear infections we mean infections of the middle ear, the space between the eardrum and the oval window which contains the ossicles. (These infections are properly called otitis media). But the outer ear can also be infected; in practice, this usually refers an infection of the wall of the ear canal, as this is more likely to cause problems than an infection of the pinna. However, any infection of the outer ear can be considered external otitis (AKA otitis externa).
Any viral, bacterial, or fungal infection of the skin of the pinna or ear canal is external otitis. A very common cause of external otitis is swimming in polluted waters, so the term Swimmer's Ear is often used for cases of indeterminate origins, and may be used indiscriminately by doctors who want to communicate to their patients in familiar terms. However, there are some common culprits in external otitis, most particularly the herpes simplex virus and viral bullous myringitis, which often spreads from the middle ear through (or around) the eardrum.
The most common symptoms are inflammation, irritation and itching, drainage of pus, and redness of the affected area. But you know what an infection looks and feels like; use your judgment. If untreated the infection can spread into the surrounding cartilage, bone, or the middle ear, and may result in Very Very Serious Infections and/or hearing loss.
While there are many possible causes of infection, swimming in unclean water, scratching the skin while trying to clear out ear wax (and yes, even Q-tips can cause this), and allowing infections on/in your middle ear or face to go untreated are common causes. Allergies and chronic skin conditions may mimic external otitis. When in doubt, go see your doctor!