The ear canal is the passage that leads from the outer world into the eardrum. Technically, it starts at the external auditory meatus at the base of the pinna and continues to the eardrum, which is technically called the tympanic membrane.
The ear canal has two sections. The outer two thirds of the canal is cartilage covered by a layer of epithelium; this is the section that has ear hairs and the glands that create ear wax (cerumen), which protect the canal from outside intrusions. The ceruminous glands secrete oils that are noxious to insects and other potential intruders, and that will thicken and dry out into earwax, which will slowly crumble and fall out of the ear on its own. Wax buildup is often the result of people using Q-tips which push the wax down against the eardrum, and may also add cotton fibers to the mix, which act to help bind the wax into a mass. Of course, some individuals just produce too much wax, and will need to wash it out of their ears using an ear wax removal kit.
The inner third of the ear canal passes through the bone of the skull. The epithelium that lines the ear canal is continuous over the eardrum, so in a healthy ear there is no passage between the middle ear and the outer ear. However, direct trauma to the eardrum can puncture it, and a severe ear infection can eat through the eardrum (this process is aided by increased pressure in the middle ear as fluid builds up; this pressure is responsible for most of the pain involved). If the eardrum is pierced pus and mucus may drain out through the ear canal, and this is perfectly fine. It is not uncommon for a doctor to pierce the eardrum and insert a tympanostomy tube (commonly called T-tubes or simply tubes) to allow ear infections to drain. The eardrum and epithelium will heal themselves over time.
The ear canal is about 2.5 cm long and 5 to 7 mm wide; this space has a resonant frequency of approximately 2500 Hz, having the effect of amplifying these frequencies; other frequencies important for understanding human speech are amplified in the pinna and the middle ear.
While it is not particularly common to have diseases that specifically affect the ear canal, there are a few; congenital deformities (such as stenosis and atresia of the ear canal), external otitis (including Swimmer's Ear), wax buildup, cysts, tumors (particularly osteomas), and bony exostoses (AKA Surfer's Ear) are all possibilities. By far the most likely causes of hearing loss affecting the ear canal are wax buildup and foreign objects (beads, beans, insects, etc.) lodged in the ear canal. The ear wax you can remove yourself (not with a Q-tip!), but any other objects should be removed by a professional.